chore: move Quarkus platform PR info to the project.yml file

[xstefank]

Fixes #996

[xstefank]

@metacosm design is open for discussion. Metadata extension only takes the leaf name of the yaml, so any structure in project.yml is ignored.

[metacosm]

There needs to be potentially several target branches as a platform PR might need to be opened against main but also the current stable version (for example, right now, main and 3.17 potentially)

[metacosm]

I guess this shouldn't be needed anymore if we specify branches to target explicitly

[metacosm]

@metacosm design is open for discussion. Metadata extension only takes the leaf name of the yaml, so any structure in project.yml is ignored.

Ah, too bad. In that case, the leaf should probably contain quarkus-platform in some form in its name to avoid potential collisions?

[xstefank]

@metacosm these don't need to be strings, but otherwise we will get warnings that we mix strings and numbers. I mean, it can also be without quotes like this if preferred:

  quarkus-platform-branches:
    - main
    - 3.17