MDBF-1156: Add Ubuntu 26.04 - ci_images add galera

[grooverdan]

With MDEV-38978 resolved, there is now mariadb-4.x galera packages for 26.04 in both dev and prod BB environments.

This adds galera to the bb-worker images.

Note the 26.04-amd64v3 image will gain an non-amd64v3 galera package for now. There currently isn't a
galera-amd64v3 builder and changes to
ci_build_images/debian.Dockerfile needs to happen to include the variant package.

Note: I did test building galera-4 using ./scripts/build -p on the amd64v3 bb worker and it built and unit tested ok.

[RazvanLiviuVarzaru]

@grooverdan the CI workflow is trying to get Galera from production i.e. ci.mariadb.org
For this to happen we will have to wait to release the builders in production, re-trigger galera builds, then we can update these workflows.

If you want galera testing on the V3 builder, I can do "some tricks" to make it happen,
and have at least a 11.8 run in DEV but this PR will still have to wait for a production deployment.

[grooverdan]

yep, a fix into the v3 builder is quite sufficient to capture any regressions.