Tests: Use Ubuntu 22.04 runner for PHP 7.0 tests

[tbradsha]

This uses the tests matrix to pass through ubuntu-latest as the default runner, with an override to use the ubuntu-22.04 runner for PHP 7.0 tests.

Proposed changes:

As of yesterday, our tests on trunk started timing out:

https://github.com/Automattic/jetpack/actions/workflows/tests.yml?query=branch%3Atrunk

We use ubuntu-latest in our workflows, and that runner is now using Ubuntu 24.04:

actions/runner-images#10636

For some reason, PHP 7.0 + Ubuntu 24.04 has major network issues, which results in double the time to run (and in most cases, a timeout). Given nothing obvious from our end is causing this and support for PHP 7.0 will be dropped with WP 6.7 (mid-November), it's easiest to just switch to the old runner (ubuntu-22.04) for PHP 7.0 tests.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Verify the PHP 7.0 tests are on the ubuntu-22.04 runner, e.g.:
https://github.com/Automattic/jetpack/actions/runs/11354027780/job/31580361443?pr=39779

[github-actions]

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[anomiex]

Looks reasonable and seems to work.