[DPE-8550] Check juju version before removing revision and remove discourse charmers test

[dragomirp]

• Check juju version before removing a secret revision
• Remove legacy admin relation test

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 73.34%. Comparing base (f660c4f) to head (6a2ee81).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
- Coverage   73.34%   73.34%   -0.01%     
==========================================
  Files          15       15              
  Lines        3913     3916       +3     
  Branches      573      574       +1     
==========================================
+ Hits         2870     2872       +2     
- Misses        829      830       +1     
  Partials      214      214              

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[dragomirp]

The discourse charmers charm has not been updated since 2021, is superseded by a newer charm and fails to deploy locally and on CI. I don't think there's another db-admin charm to use, so removing the test.