[release-21.0] SwitchTraffic: use separate context while canceling a migration (#17340)

[vitess-bot]

Description

This is a backport of #17340

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

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If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
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Backward compatibility

• Protobuf changes should be wire-compatible.
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• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Project coverage is 67.52%. Comparing base (656f35c) to head (baa3c65).
Report is 1 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/traffic_switcher.go	0.00%	16 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17366      +/-   ##
================================================
+ Coverage         67.49%   67.52%   +0.03%     
================================================
  Files              1570     1570              
  Lines            251411   251421      +10     
================================================
+ Hits             169680   169784     +104     
+ Misses            81731    81637      -94     

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