[Feature] Add deleted filename to retention enforcement logging

[jessecowens]

Is your feature request related to a problem? Please describe.
Due to a strange issue reported in the WordPress forums, I'd like to see the deleted filename added to the retention enforcement logging. This could help determine if the most recently added backup is being deleted instead of the old ones.

Describe the solution you'd like
In the log file, add a line indicating the filename of the deleted file:

[2021-01-13 13:35:32 UTC] Enforcing retention…
[ADD A LINE HERE     UTC] Deleted archive file boldgrid-backup.....zip (Last modified timestamp here)
[2021-01-13 13:35:32 UTC] Retention enforcement complete!