[Disaster Recovery] Upgrade compatibility checklist for 26.4

Part of https://github.com/cockroachdb/cockroach/issues/175019 for DB teams to go through the upgrade compatibility checklist for 26.4.

## Upgrade compatibility requirements for 26.4
26.4 must be upgradeable **from both 26.3 and 26.2**

## Checklist Sign-off for 26.4

## Checklist
_[from Confluence: [26.4 Upgrade Compatibility Checklist](https://cockroachlabs.atlassian.net/wiki/spaces/RE/pages/6012665948/26.4+Upgrade+Compatibility+Checklist). Tip: one way to review the checklist could be to integrate it into the team pre-mortem, with EMs and TLs signing-off that this has been reviewed with the team.]_

### New Functionality:

- [ ] Has upgrade compatibility from both 26.3 and 26.2 been considered in your team pre-mortem?
- [ ] Has the team introduced any migrations or version gates?
- [ ] Have you tested all your migrations in upgrades from 26.3 and 26.2?
- [ ] Have you tested all your version gates and related logic?
- [ ] Has upgrade and mixed-version functionality (mixed versions including 26.2, 26.3 and 26.4) been tested against new functionality [1]?

### Existing Functionality:

- [ ] Have you pulled out any 26.3 code (since April 2026) that could potentially impact upgrades or mixed-version functionality?
- [ ] (if so, has this been resolved to support upgrade compatibility?)
- [ ] Have you made any changes to existing functionality that could misbehave on a cluster created at an arbitrarily old version (either after upgrade to 26.4, or in a mixed cluster with some 26.2 or 26.3 nodes)?

[1] Note that there are a ton of other ways to break upgrades, ways that may not even use any migrations or version gates. e.g. Storage team recently made a fix to the Pebble comparator (related to a now obsolete flag) which lead to inconsistent replica crashes on a 24.2 cluster

Jira issue: CRDB-68200