Problem: MySQL authentication method may remain as `mysql_native_password` in upgrades from 2.6

AtoM 2.6.x required to use the legacy authentication method in MySQL in order to work with PHP 7.2:

https://accesstomemory.org/en/docs/2.6/admin-manual/installation/linux/ubuntu-bionic/#mysql

This is no longer needed in 2.7 or 2.8 with PHP 7.4 but some users upgrading the system over the same MySQL instance may still have that configuration change. As noted by Jim Adamson in the following user forum thread, this makes the MySQL log grow with warnings about the deprecation of such method:

https://groups.google.com/g/ica-atom-users/c/VGcUg43yWrU/m/vIoWWFh0AgAJ

As Jim suggests in that thread, we could add a note to alter existing AtoM users in the MySQL server in the upgrading docs for 2.7 and 2.8:

https://accesstomemory.org/en/docs/2.7/admin-manual/installation/upgrading/#copy-your-old-data

```
ALTER USER 'username'@'localhost' IDENTIFIED WITH caching_sha2_password BY 'password';
```

Read the 2.7 and 2.8 upgrading documentation, especially the “Copy your old data” section, and compare it with the MySQL installation guidance for 2.6. Add a note explaining how to update existing AtoM users from mysql_native_password to caching_sha2_password, including the provided ALTER USER example; done means both upgrade guides clearly cover this case.

Written by the indexing model from the issue text.