Add explicit versions in the branches.json and use them for calculating unreleased versions for BwC tests

[jozala]

Work in progress

Gradle task added to modify branches.json. Possibility to add, remove and update branches in the file with the version field added next to the branch.
After the change the file looks like this:

{
    "branches": [
        {
            "branch": "main",
            "version": "9.1.0"
        },
        {
            "branch": "9.0",
            "version": "9.0.3"
        },
        {
            "branch": "8.19",
            "version": "8.19.1"
        }
    ]
}

BwC test configuration has been changed to use changed branches.json file to calculate unreleased ES versions, instead of combining data from Version.java and branches.json.

[breskeby]

referencing project here while executing task is deprecated. In this case I think we should just inject ProjectLayout instead of project in the ctor and then use layout.getSettingsDirectory().file('branches.json').asFile.

The Util.locateElasticsearchWorkspace is just a utility method only used to resolve files relateive from internal builds within this repository, which we don't need here as we never run this task anywhere else but in the :elasticsearch root build

[breskeby]

Also we probably want to make this file an outputFile property of the task

[jozala]

I've now changed the way to reference branches.json file location using ProjectLayout and also made the branches file an @OutputFile property of the task.

[breskeby]

Usually we do not use that annotation. In Gradle tasks, you usually mark properties that are optional as @optional and as @input

[jozala]

I've made all three parameters @Input and @Optional.

[jozala]

There is still a problem with GlobalBuildInfoPluginSpec failing because BwcVersions directly use static version from VersionProperties. It needs to be solved before merging to main.