release-julia-1.13-new: build matrix + job-set specialization

[Keno]

This is the sole release-1.13-specific commit layered on top of master. The release-julia-1.13 branch is now master + this commit, so the shared OIDC/KMS CI architecture (#544) stays byte-identical to master and future master -> release-* cherry-picks apply cleanly, per review feedback on #564.

Everything release-1.13-specific lives here:

• Build/test matrix (pipelines/**/platforms/*.arches): pin the rootfs images to v6.00 / docker v7.10, put mmtk on the package_linux image, and drop master's Windows -Wno-error CFLAGS -- i.e. keep release-1.13's existing build environment rather than master's (1.14) one.

• Job set (utilities/render_launch_pipeline.py): omit the JuliaSyntax and JuliaC launch groups, which are new on master and not part of release-1.13 CI. The juliasyntax.* / juliac/ YAMLs stay in-tree (from master), so either group re-enables by re-adding its blocks.append(...).

NOT yet validated on Buildkite.

[Keno]

@DilumAluthge See if you like this better. That said, we may need to bump the rootfs images anyway if we want the aws upload to work on julia-pr, but let's start here.

[Keno]

Ok, so my plan is merge this right away (it's a new branch), try it out on the PR, if it works, swap the branch names.

[DilumAluthge]

Sounds good. Then future commits to master can easily be cherry-picked ("forward ported", I suppose) to the release-1.13 branch.

I wouldn't delete the old release-1.13 branch, just rename it to something like release-Julia-1.13-old, so that we keep the git history around.