[bwc] Add bugfix3 project

[brianseeders]

There are now 3 "bugfix" projects from a BWC standpoint, which causes the following error:

./gradlew --quiet --console=plain tagVersions --release=8.18.0 --tag-version=TransportVersion:8840002 --tag-version=IndexVersion:8525000
...
        | A problem occurred evaluating project ':distribution:bwc'.
        | > Failed to apply plugin 'elasticsearch.internal-distribution-bwc-setup'.
        |    > Cannot add extension with name 'bwcSetup', as there is an extension already registered with that name.

Because there are two bugfix2 projects. This adds a 3rd one. I'm not sure if this is the best approach here.

Finalizing the 8.18.0 release is blocked until this is solved.

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[jozala]

How about removing the 7.17 from branches.json instead?
The outcome would be then:

8.17.6=UnreleasedVersionInfo[version=8.17.6, branch=8.17, gradleProjectPath=:distribution:bwc:maintenance],
8.18.1=UnreleasedVersionInfo[version=8.18.1, branch=8.18, gradleProjectPath=:distribution:bwc:bugfix2],
8.19.0=UnreleasedVersionInfo[version=8.19.0, branch=8.x, gradleProjectPath=:distribution:bwc:minor],
9.0.1=UnreleasedVersionInfo[version=9.0.1, branch=9.0, gradleProjectPath=:distribution:bwc:bugfix],
9.1.0=UnreleasedVersionInfo[version=9.1.0, branch=main, gradleProjectPath=:distribution]

From the BwC tests I believe it makes sense because we do not test the compatibility for upgrades between 7 and 9.
Version 7.17 would be still tested in BwC tests on 8.x branches.

I am not sure what might be the other side-effects of removing 7.17 from branches.json. I know we still maintain this branch, so some tools might depend on that. Can you advise on that?

[jozala]

Alternatively, it would be possible to filter the featureFreezeBranches to exclude 7.17 from the logic of BwC but leave it in branches.json:

        // Now handle all the feature freeze branches
        List<String> featureFreezeBranches = developmentBranches.stream()
            .filter(b -> Pattern.matches("[0-9]+\\.[0-9]+", b))
            .filter(b -> currentVersion.getMajor() - Version.fromString(b, Version.Mode.RELAXED).getMajor() <= 1) // I've added this line
            .sorted(reverseOrder(comparing(s -> Version.fromString(s, Version.Mode.RELAXED))))
            .toList();

It gives the same result as removing 7.17 from branches.json:

8.17.6=UnreleasedVersionInfo[version=8.17.6, branch=8.17, gradleProjectPath=:distribution:bwc:maintenance],
8.18.1=UnreleasedVersionInfo[version=8.18.1, branch=8.18, gradleProjectPath=:distribution:bwc:bugfix2],
8.19.0=UnreleasedVersionInfo[version=8.19.0, branch=8.x, gradleProjectPath=:distribution:bwc:minor],
9.0.1=UnreleasedVersionInfo[version=9.0.1, branch=9.0, gradleProjectPath=:distribution:bwc:bugfix],
9.1.0=UnreleasedVersionInfo[version=9.1.0, branch=main, gradleProjectPath=:distribution]

It also have no side-effects and should be compatible with other branches so we can backport it to 8.x branches.
What do you think?
I will prepare a separate PR with that so we can decide which path to go.

EDIT:
PR prepared with alternative approach here: #126895