[DPE-8395] Remove secret's old revision

[marceloneppel]

Issue

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

Solution

Port of #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 64.57%. Comparing base (3a00ea4) to head (b709081).
⚠️ Report is 1 commits behind head on 16/edge.

❌ Your project status has failed because the head coverage (64.57%) 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    #1207      +/-   ##
===========================================
+ Coverage    64.53%   64.57%   +0.04%     
===========================================
  Files           17       17              
  Lines         4339     4344       +5     
  Branches       671      671              
===========================================
+ Hits          2800     2805       +5     
  Misses        1357     1357              
  Partials       182      182              

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