[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 67.38%. Comparing base (6317a84) to head (ad85559).
⚠️ Report is 1 commits behind head on 16/edge.

❌ Your project status has failed because the head coverage (67.38%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1128      +/-   ##
===========================================
+ Coverage    67.34%   67.38%   +0.04%     
===========================================
  Files           18       18              
  Lines         3947     3952       +5     
  Branches       564      564              
===========================================
+ Hits          2658     2663       +5     
  Misses        1116     1116              
  Partials       173      173              

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