[DPE-8498] Remove secret's old revision

[marceloneppel]

Issue

The charm does not remove the old revisions of a secret after updating it.

Solution

Port of canonical/postgresql-operator#1195.

Implement the secret-remove event handler.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.31%. Comparing base (cca6e6a) to head (04dd300).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1127      +/-   ##
==========================================
+ Coverage   73.28%   73.31%   +0.03%     
==========================================
  Files          15       15              
  Lines        3904     3909       +5     
  Branches      573      573              
==========================================
+ Hits         2861     2866       +5     
  Misses        829      829              
  Partials      214      214              

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