[Tracking Issue] Upgrade compatibility checklist for 26.4

Tracking issue for DB teams to go through the [upgrade compatibility checklists for 26.4](https://cockroachlabs.atlassian.net/wiki/spaces/RE/pages/6012665948/26.4+Upgrade+Compatibility+Checklist):

- https://github.com/cockroachdb/cockroach/issues/175056 (CDC)
- https://github.com/cockroachdb/cockroach/issues/175057 (DB Server)
- https://github.com/cockroachdb/cockroach/issues/175058 (Disaster Recovery)
- https://github.com/cockroachdb/cockroach/issues/175059 (KV)
- https://github.com/cockroachdb/cockroach/issues/175061 (Observability)
- https://github.com/cockroachdb/cockroach/issues/175062 (Product Security)
- https://github.com/cockroachdb/cockroach/issues/175063 (Specialized Indexing)
- https://github.com/cockroachdb/cockroach/issues/175064 (SQL Foundations)
- https://github.com/cockroachdb/cockroach/issues/175065 (SQL Queries)
- https://github.com/cockroachdb/cockroach/issues/175066 (Storage)
- https://github.com/cockroachdb/cockroach/issues/175067 (Supportability)
- https://github.com/cockroachdb/cockroach/issues/175068 (Test Engineering)

## Exempt teams

These teams have explicitly confirmed that they do not deliver code that impacts upgrade compatibility between Regular releases, and do not have a team checklist:

- Migrations ([confirmed by Eric Alton, Sept 2026](https://cockroachlabs.slack.com/archives/CNRNN77ST/p1789158844193039?thread_ts=1789158460.763239&cid=CNRNN77ST))

Jira issue: CRDB-68170

Start by reading the linked 26.4 Upgrade Compatibility Checklist, then review the child issues for CDC, DB Server, Disaster Recovery, KV, Observability, Security, Indexing, SQL, Storage, Supportability, and Test Engineering. Done means the listed teams have completed their compatibility checks and the tracking issue reflects their status; Migrations is explicitly exempt.

Written by the indexing model from the issue text.