Remove mysql 5.7 tests that are no longer required

[GuptaManan100]

Description

Since Vitess has deprecated support for MySQL 5.7, we can remove some of the tests.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required - Remove MySQL 5.7 from supported versions for backup website#1735

Deployment Notes

[vitess-bot]

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[mattlord]

Thank you, @manan! Any reason that we can't remove the OnlineDDL 5.7 workflows? cc @shlomi-noach Any reason that we can't remove the 5.7 unit test workflow? These are generally MUCH more costly than the ones we're currently removing.

IMO now is a good time to hash out what our general policy is on 5.7 CI testing so that it's clear what can be removed and when, noting that in the issue description, and then applying that to all of the 5.7 specific workflows.

[deepthi]

Any reason that we can't remove the 5.7 unit test workflow?

We still support importing (MoveTables) from MySQL 5.7, and you can switch vtgate traffic back and forth during that process. So query serving needs to continue working with 5.7.
All management features are no longer supported, but external tablets are.

[deepthi]

I'll leave it up to @GuptaManan100 and @shlomi-noach whether to remove the Online DDL tests in this PR, or do a separate one.

[deepthi]

I went ahead and removed the deleted workflows from Required checks for the main branch. Note that if there are more changes to remove Online DDL workflows, branch protection rules will need to be edited again.

[shlomi-noach]

I actually went ahead and removed these:

	case strings.HasPrefix(clusterName, "onlineddl_"):
		return allMySQLVersions
	case clusterName == "schemadiff_vrepl":
		return allMySQLVersions
	case clusterName == "tabletmanager_tablegc":
		return allMySQLVersions

which now leaves zero generated tests that run on 5.7.

Elaborating:

• I see no reason to support Online DDL for 5.7 if Vitess does not support 5.7 (even if it does support MoveTables to import from 5.7)
• Therefore also the schemadiff tests can be removed
• Likewise tablegc 5.7 tests are only relevant to 5.7 instances and can be removed (nothing to impact imports or anything).

I've removed the Required flag from related CI workflows.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.42%. Comparing base (ee6b837) to head (75819fa).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15809      +/-   ##
==========================================
- Coverage   68.43%   68.42%   -0.02%     
==========================================
  Files        1558     1559       +1     
  Lines      195971   196496     +525     
==========================================
+ Hits       134121   134446     +325     
- Misses      61850    62050     +200     

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