[DPE-6053] Fix ports

[marceloneppel]

Issue

The psotgresql-k8s K8s service is deleted when scaling down the charm application.

Solution

Use self.unit.set_ports(): canonical/observability-libs#101 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.46%. Comparing base (7da9c8c) to head (e9e1e99).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #803      +/-   ##
==========================================
- Coverage   75.48%   75.46%   -0.03%     
==========================================
  Files          12       12              
  Lines        3121     3118       -3     
  Branches      475      475              
==========================================
- Hits         2356     2353       -3     
  Misses        622      622              
  Partials      143      143              

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