Make skippable_job_adapters a working per-adapter opt-in #3118

config.rails.skippable_job_adapters becomes the switch for opting an adapter into ActiveJob-first: remove the adapter from the list to opt in. Today backend gems append their entry after Sentry.init runs, so removing it has no effect.

Backend gems should declare their default entry, so the user's config can override it. Existing setups must behave exactly as before.