Missing linux capabilities on target host running old kernel / btrfs-progs

I'm using btrfs on my Notebook which has Ubuntu 20.04 LTS with kernel 5.15. For backups I have setup an image file with btrfs on a server with Ubuntu 18.04 LTS with kernel 5.4.0. After a fresh backup btrbk-verify shows some differences:

```
[rsync] .f........x usr/lib/i386-linux-gnu/libgstbase-1.0.so.0.1602.0 # FAIL ndiffs=1
[rsync] .f........x usr/lib/ubiquity/target-config/50gkd-caps # FAIL ndiffs=2
[rsync] .f........x usr/lib/x86_64-linux-gnu/libgstcontroller-1.0.so.0.1602.0 # FAIL ndiffs=3
[rsync] .f........x usr/share/doc/iputils-ping/copyright # FAIL ndiffs=4
[rsync] .f........x usr/share/doc/iputils-tracepath/copyright # FAIL ndiffs=5
[rsync] .f........x usr/share/doc/mtr-tiny/copyright # FAIL ndiffs=6
```

On the server side (as an example):

```
getcap usr/share/doc/iputils-ping/copyright
usr/share/doc/iputils-ping/copyright = cap_net_raw+ep
```

Different files give different capabilities but on my notebook getcap gives nothing on these files. Note that `/usr/bin/ping` has the usual capabilities also on my notebook, so it is not always nothing.

On both machines the mount options for btrfs are the same. So why shows my older system capabilities, especially on docs? It is obviously not a btrbk bug but maybe someone here had similar experiences?

Start with the btrbk-verify output and compare the reported files on the Ubuntu 20.04 and 18.04 hosts using getcap, as described in the issue. Determine whether the differing file capabilities come from the kernel, btrfs, or the installed packages; done means documenting the cause and any relevant verification behavior or required project change.

Written by the indexing model from the issue text.