desktop client 6.0.0 and 6.0.0-rc1 not starting on Ubuntu 20.04

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Describe the bug

The desktop client complains about not finding GLIBCXX_3.4.29 and GLIBCXX_3.4.30

I think that these are newer GLIBC versions than are usually available on Ubuntu 20.04 (which is getting a bit old now).

The output is:

$ /usr/bin/owncloud
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libowncloudGui.so)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Xml.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Xml.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libownCloudLibSync.so.0)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Widgets.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Widgets.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Quick.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Quick.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6QmlMeta.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6QmlModels.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6QmlModels.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /lib/x86_64-linux-gnu/libgcc_s.so.1: version `GCC_12.0.0' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Gui.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Gui.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Gui.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6DBus.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Qml.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Qml.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Network.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Network.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /lib/x86_64-linux-gnu/libgcc_s.so.1: version `GCC_12.0.0' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Core.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Core.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Core.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Core.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libappimageupdate.so)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libappimageupdate.so)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libzsync2.so)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6Concurrent.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6QuickTemplates2.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libQt6QuickTemplates2.so.6)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libicuuc.so.76)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libcpr.so.1)
/opt/owncloud-client.AppDir/usr/bin/owncloud: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /opt/owncloud-client.AppDir/usr/bin/../lib/libcpr.so.1)

Is Ubuntu 20.04 still supposed to be supported?

I have tried to install later GCC on my Ubuntu 20.04, but with no joy yet. Are we required to install some later/newer things to get the desktop client working in Ubuntu 20.04?

Expected behavior

The client should run.

Steps to reproduce the issue

Have an Ubuntu 20.04 system.

/usr/bin/owncloud

See the messages about GLIBCXX_3.4.29 and GLIBCXX_3.4.30

Screenshots

No response

Logs

No response

Client version number

Happens with all 6.0.0 and 6.0.1 images that have been released up to now.

Desktop environment (Linux only)

Ubuntu 20.04

Client package version and origin (Linux only)

No response

Installation path (Windows only)

No response

Server information

No response

Additional context

No response

Start by reproducing the launch failure with /usr/bin/owncloud on Ubuntu 20.04 using the 6.0.0 or 6.0.1 images, then inspect the reported AppDir libraries and missing GLIBCXX, GCC, and CXXABI versions. Done means the desktop client starts on Ubuntu 20.04 without these library-version errors.