[Migrations] Upgrade compatibility checklist for 26.2

[celiala]

Part of #165467 for DB teams to go through the upgrade compatibility checklist for 26.2.

Upgrade compatibility requirements for 26.2

26.2 must be upgradeable from both 26.1 and 25.4

Checklist Sign-off for 26.2

Checklist

[from Confluence: 26.2 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 26.1 and 25.4 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 26.1 and 25.4?
• Have you tested all your version gates and related logic?
• Has upgrade and mixed-version functionality (mixed versions including 26.1, 25.4 and 26.2) been tested against new functionality [1]?

Existing Functionality:

• Have you pulled out any 25.4 code (since October 2025) 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 26.2, or in a mixed cluster with some 26.1 or 25.4 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-61400