[DPE-8490] Use non-root for PostgreSQL K8s charm

[taurus-forever]

Issue

Solution

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 68.64%. Comparing base (24cce03) to head (d320b25).
⚠️ Report is 2 commits behind head on 16/edge.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1347      +/-   ##
===========================================
+ Coverage    68.56%   68.64%   +0.07%     
===========================================
  Files           16       16              
  Lines         3868     3881      +13     
  Branches       590      590              
===========================================
+ Hits          2652     2664      +12     
- Misses        1008     1009       +1     
  Partials       208      208              

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