adapter: always apply workload class on ALTER CLUSTER

[teskje]

sequence_alter_cluster_stage had a couple paths where the supplied workload class was not applied to the controller. As a result, metrics for production clusters sometimes didn't get their workload_class label applied correctly. This is fixed in this commit by making sure the workload class is always updated in the controller, even when a cluster finalization is needed.

Looks like this got broken a while ago in #27904.

Motivation

• This PR fixes a recognized bug.

Fixes https://github.com/MaterializeInc/database-issues/issues/8941

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.