cleanup: follow-up tasks for monorepo structure

[embik]

We have migrated successfully to a monorepo structure, but #3638 was just a bandaid for a couple of things. We have to finalize the migration by doing the following things:

• Stop ignoring staging repositories for the boilerplate check and instead run boilerplate checks on all of them.
• Test and validate module version replacements done by publishing-bot to ensure that modules depend on the right versions of each other.
• Stop ignoring staging repositories for linting.
• Remove leftover artifacts from staging repositories (e.g. .prow.yaml files).
• Ensure that test pipelines cover staging repositories (to substitute adequately for the prow jobs that covered them previously).
• bug: sdk is not go install'able anymore #3653

If you find more, please consider adding it here.