Issue #903: update to Checkstyle 12.1.0

[vtintillier]

Fixes #903

[Calixte]

Our P2 repo is massive and GitHub Pages chokes on it.
This one won't go through: https://github.com/checkstyle/eclipse-cs-update-site/actions/runs/19341048508
I will have to fix this before we can move forward.

[vtintillier]

Looks like we ship a lot of dependencies in our update site since #475. Not sure it was intended to include so much...

[Calixte]

Actually, the releases are 16MB smaller since then:

% du -hd 1 releases | sort -k2 -V | tail
122M	releases/10.25.0.202508191905
122M	releases/10.25.1.202508221751
122M	releases/10.26.0.202508262013
122M	releases/10.26.1.202508262045
119M	releases/11.0.0.202508271907
119M	releases/11.0.1.202509241330
119M	releases/11.1.0.202510311516
103M	releases/12.0.0.202511121607
103M	releases/12.0.1.202511131535
103M	releases/12.1.0.202511131746

[Calixte]

Our p2 repository is 8GB.
The problem is with the GitHub action runner which hits the disk space limit during deployment.
Maximum recommended GitHub Pages size is 1GB (hard max is 10GB) so we're lucky it has been working this whole time.
We could delete older versions of the plugin from the p2 repository as we add new ones. Or we could make another "legacy" repository and move them there – though it may be too much work for what it's worth.
@romani thoughts?

Note that the plugin size increase from 34MB to 110MB with version 10.5.0 so it would be nice to bring this back down eventually.

[Calixte]

Looks like we ship a lot of dependencies in our update site since #475. Not sure it was intended to include so much...

I'm sorry, I misread and thought you meant #385, but yeah, #475 is definitely when the size exploded.

[vtintillier]

I think the intent was to add some bundles that are automatically wrapped from maven artifacts during the build, but that a quick fix was to just include all dependencies. Maybe we can just list those explicitly to be included in the p2 repo.

Also, we already have the major version number in the p2 url when installing the plugin. Maybe we could instead have one GitHub repository per major version?

[Calixte]

Maybe we can just list those explicitly to be included in the p2 repo.

This seems to do the trick: #915

Maybe we could instead have one GitHub repository per major version?

I worry this would be too much overhead. @romani Any thoughts?