[DPE-7726] Use Patroni API for is_restart_pending

[dragomirp]

Partial port of #1049

Use the Patroni API to detect restart need

Checklist

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

[codecov]

Codecov Report

❌ Patch coverage is 65.21739% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.70%. Comparing base (7542643) to head (aebd98d).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/charm.py	53.57%	12 Missing and 1 partial ⚠️
src/cluster.py	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1165      +/-   ##
==========================================
- Coverage   75.76%   75.70%   -0.06%     
==========================================
  Files          16       16              
  Lines        4163     4174      +11     
  Branches      631      628       -3     
==========================================
+ Hits         3154     3160       +6     
- Misses        786      793       +7     
+ Partials      223      221       -2     

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[dragomirp]

We should most probably update Patroni before we can port this.