[Test Engineering] 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-68210

Start with parent issue 175019 and the 26.4 Upgrade Compatibility Checklist linked in the issue. Review the checklist with the relevant DB teams, covering upgrades from 26.2 and 26.3, migrations, version gates, mixed-version behavior, and existing functionality; done means the checklist is completed and team sign-off is recorded.

Written by the indexing model from the issue text.