github/workflows: remove contrib workflow

[ivanvc]

This job is already running in the Prow infrastructure.

After merging kubernetes/test-infra#34335 it will be blocking, so we can remove this workflow.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.95%. Comparing base (14cf669) to head (0bbc42a).
Report is 4 commits behind head on main.

Additional details and impacted files

see 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19421      +/-   ##
==========================================
- Coverage   68.95%   68.95%   -0.01%     
==========================================
  Files         420      420              
  Lines       35753    35753              
==========================================
- Hits        24655    24654       -1     
+ Misses       9677     9673       -4     
- Partials     1421     1426       +5     

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[ivanvc]

/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, ivanvc

The full list of commands accepted by this bot can be found here.

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• .github/OWNERS [ahrtr,ivanvc]

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[ivanvc]

/cherry-pick release-3.6

[k8s-infra-cherrypick-robot]

@ivanvc: new pull request created: #19549

In response to this:

/cherry-pick release-3.6

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