K8s: remove ephemeral secrets implizite via GC and explizite

[6543]

try to close #6740

[codecov]

Codecov Report

❌ Patch coverage is 50.84746% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.21%. Comparing base (e3f827b) to head (7625ff5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pipeline/backend/kubernetes/kubernetes.go	22.72%	12 Missing and 5 partials ⚠️
pipeline/backend/kubernetes/secrets.go	67.56%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6762      +/-   ##
==========================================
+ Coverage   47.00%   47.21%   +0.20%     
==========================================
  Files         438      438              
  Lines       29458    29505      +47     
==========================================
+ Hits        13847    13931      +84     
+ Misses      14447    14391      -56     
- Partials     1164     1183      +19     

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