Repair wp/5.6 branch Actions: Ignore the stale Browserslist data warning in unit tests - #84967

Part of #82937.

What?

Sets BROWSERSLIST_IGNORE_OLD_DATA=true before Jest boots, in test/unit/jest.config.js.

Why?

test/unit/jest.config.js uses @wordpress/jest-preset-default, whose @wordpress/jest-console assertions fail any test that logs a warning. packages/babel-preset-default and packages/browserslist-config log "Browserslist: caniuse-lite is outdated" during the run, so the JavaScript job fails two suites:

FAIL packages/babel-preset-default/test/index.js
  ● Babel preset default › transpilation works properly
FAIL packages/browserslist-config/test/index.test.js
  ● should not contain invalid queries

    expect(jest.fn()).not.toHaveWarned(expected)
    ["Browserslist: caniuse-lite is outdated. Please run:
    npx browserslist@latest --update-db"]

This branch pins browserslist@4.14.0 and an equally old caniuse-lite. The warning depends on the data's age against today's date, so it will keep appearing on a frozen branch regardless of code changes.

At this point in the stack the job stops earlier, at the npm ci failure fixed by #84968; the warning failure shows once that's fixed.

How?

Ports wp/7.0's a468930 ("Tests: Ignore Browserslist old data warning in unit tests"). It doesn't cherry-pick cleanly because this branch's config has no process.env.TZ line to anchor to, so the line goes after the transpiledPackageNames block, as on wp/5.9 (#84408) and wp/6.0 (#84275).

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.