Use release branches instead of git tags for upgrade/downgrade

[frouioui]

Description

This PR changes what git ref we use during the upgrade/downgrade tests. Instead of using the next/previous git tags, we will use release branches. This will ensure that a change made on main is compatible with the next release we will make out of release-20.0. Prior to that, we were just checking that a branch was compatible with the next/previous release, which does not cover new code that was merged on the release branch.

Let's backport this to all supported release branches as we want all branches to follow this new methodology.

Tests

# upgrade on main, no upgrade = empty line output
$ ./tools/get_next_release.sh main refs/pull/00000/merge 


# upgrade on release-20.0
$ ./tools/get_next_release.sh release-20.0 refs/pull/00000/merge
main

# upgrade on release-19.0
$ ./tools/get_next_release.sh release-19.0 refs/pull/00000/merge
release-20.0

# upgrade on release-18.0
$ ./tools/get_next_release.sh release-18.0 refs/pull/00000/merge
release-19.0

# downgrade on main
./tools/get_previous_release.sh  refs/heads/main
release-20.0

# downgrade on release-20.0
$ ./tools/get_previous_release.sh release-20.0 refs/pull/00000/merge
release-19.0

# downgrade on release-19.0
$ ./tools/get_previous_release.sh release-19.0 refs/pull/00000/merge
release-18.0

# downgrade on release-18.0
$ ./tools/get_previous_release.sh release-18.0 refs/pull/00000/merge
release-17.0

Related Issue(s)

• Part of Improve End-to-End Test Management and Upgrade/Downgrade Testing #16365

[vitess-bot]

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[GrahamCampbell]

The downside of this is that it tests release-17.0 for upgrade to 18.0, but there will never be another tag there, so probably the last 17.0 tag should be used?

[GrahamCampbell]

Unless there were to be a change to the EOL policy to always do a final release when EOLing a branch.

[frouioui]

There has not been a policy change regarding the lifecycle of our releases. That's a good point, we should probably use the tag in this situation.

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