admission: lower physical cluster replication to background work priority

**Is your feature request related to a problem? Please describe.**
PCR currently uses normal admission control priority, meaning it won't be subject to replication AC. We saw that this can lead to uncontrolled IO overload, as follower writes bypass AC:


**Describe the solution you'd like**

Lower the admission control work priority of PCR to background here:

https://github.com/cockroachdb/cockroach/blob/935d796608663c74b726ad42659cd91aa502d11b/pkg/ccl/streamingccl/streamingest/stream_ingestion_processor.go#L992-L995

Jira issue: CRDB-37721

Start in pkg/ccl/streamingccl/streamingest/stream_ingestion_processor.go around lines 992-995, where the physical cluster replication admission control work priority is set. Trace the surrounding PCR admission-control setup and verify that its priority is background so follower writes are subject to replication admission control.

Written by the indexing model from the issue text.