sanitize agent should allow done updates to canceled pipelines

[6543]

again ... :/ ...I think #6391 will help us not having regressions that much 😅 ...

what the original pull did not do... it only checked the current state without checking what the new state should switch to.
we have to verify that and allow more update paths without introducing more bugs and still have it hardened to avoid getting bugs into the state caused by rouge/buggy agents...

this also simplifies the code

cause: #6308

[lafriks]

on small comment but otherwise LGTM

[codecov]

Codecov Report

❌ Patch coverage is 83.60656% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.44%. Comparing base (a688600) to head (23f7043).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
server/pipeline/step_status.go	70.00%	5 Missing and 1 partial ⚠️
server/rpc/sanitize.go	89.47%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6394      +/-   ##
==========================================
- Coverage   35.45%   35.44%   -0.01%     
==========================================
  Files         427      427              
  Lines       28646    28650       +4     
==========================================
- Hits        10156    10155       -1     
- Misses      17561    17565       +4     
- Partials      929      930       +1     

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[6543]

note the e2e test in #6391 already catches this bug and faills before this pull ...

-> https://ci.woodpecker-ci.org/repos/3780/pipeline/33253/46