docs(migration): add v21 → v22 migration section

[just-jeb]

PR Checklist

• Tests for the changes have been added (for bug fixes / features) — N/A (docs only)
• Docs have been added / updated (for bug fixes / features)

PR Type

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

MIGRATION.MD has no v21 → v22 section. The v22 line ships breaking Jest builder behavior changes (#2191, #2212) and the new ng add / ng update schematics (#2267) with no migration guidance.

Issue Number: N/A

What is the new behavior?

Adds a Migration from version 21 to version 22 section at the top of MIGRATION.MD documenting:

• What's new — ng add (jest, custom-esbuild, custom-webpack) and ng update @angular-builders/jest. Notes the (from, to] migration window: a project on v17–v21 can jump straight to v22 in one ng update.
• Breaking (jest): isolatedModules now defaults to true (builder behavior change; ng update only advises, does not rewrite config — verified against migrations/v22).
• Breaking (jest): coverageDirectory now scoped per-project (<projectRoot>/coverage).
• custom-esbuild / custom-webpack: no breaking changes; ng add now available.
• Karma vs Vitest note: Angular 22 deprecates (does not remove) Karma; how ng add @angular-builders/jest detects/migrates Karma/Vitest/Jest setups.

Content was verified against the actual schematics implementation on feat/builder-schematics (#2267), not the original handoff draft — the v22 migration is advisory-only, which the draft had stated incorrectly.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Documentation only.

Other information

Targets release/v22. Documents behavior introduced by #2191, #2212, and #2267 — best reviewed/merged alongside that stack.