Fix Downgrade problem from v20 in semi-sync plugin

[GuptaManan100]

Description

As pointed out in #16300, specifically #16300 (comment) outlines the steps that can lead to downgrades from v20 to v19 vttablets as broken.

The problem is that in v20 we started using new semi-sync plugins even though v19 didn't support them. The fix is that we should continue to use the old semi-sync plugins in v20 while still adding the code to handle the new plugins. This way when we start using the new plugins in v21, upgrades and downgrades won't be broken.

This PR also adds a new workflow with a test that reproduces the problem and ensures that it is fixed by the changes in this PR. It also verifies that the new plugins work in v20.

No changes on main are required. The fix is v20 specific and hence the PR has only be created here.

Related Issue(s)

• Fixes vtorc/vttablet: can't downgrade from v20 to v19 #16300

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.65%. Comparing base (795a7e5) to head (f71e5e0).
Report is 1 commits behind head on release-20.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16357      +/-   ##
================================================
- Coverage         68.66%   68.65%   -0.01%     
================================================
  Files              1541     1541              
  Lines            197830   197827       -3     
================================================
- Hits             135833   135825       -8     
- Misses            61997    62002       +5     

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[frouioui]

Shouldn't we also have that test on main to avoid future changes that are incompatible?

[deepthi]

Good question. I thought about this too, but it seemed like we won't be making any more changes to semi-sync config on main.

[GuptaManan100]

I can add a workflow to main, but it isn't all that necessary in my opinion. But we can always add a test similar to this on main too.

[frouioui]

I feel like we probably should, we already have the code for the test, running a new workflow won't be very expensive, and will only increase our test coverage.

[GuptaManan100]

Cool, I'll open a PR against main that adds a similar check there 👍

Edit: Here is the PR for the test on main - #16372

[frouioui]

Let's remember to mark this new workflow as required when we merge this.