[release-3.6] pkg/featuregate: Return MutableFeatureGate interface in New function

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #19803

/assign ahrtr

[k8s-ci-robot]

Hi @k8s-infra-cherrypick-robot. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ahrtr]

/ok-to-test

[ahrtr]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.10%. Comparing base (9e6394e) to head (2d8e725).
Report is 2 commits behind head on release-3.6.

Additional details and impacted files

Files with missing lines	Coverage Δ
pkg/featuregate/feature_gate.go	88.96% <100.00%> (ø)

... and 59 files with indirect coverage changes

@@               Coverage Diff               @@
##           release-3.6   #19816      +/-   ##
===============================================
- Coverage        68.95%   66.10%   -2.85%     
===============================================
  Files              420      420              
  Lines            35613    35613              
===============================================
- Hits             24557    23543    -1014     
- Misses            9630    10632    +1002     
- Partials          1426     1438      +12     

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ivanvc, k8s-infra-cherrypick-robot

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