test: add playwright mobile config

[KrupaPammi]

• Added mobile WebKit steps to playwright-local config
• Updated main.yml with playwright-mobile task.
• Pinned the mobile job to ubuntu-22.04 and kept Node 16 with Playwright 1.38.1, as newer combinations were failing due to WebKit dependency and OpenSSL issues.

Note: Playwright-staging uses 1.56.1 version as it has its own package.json and so won't be affected by Node 16 but for playwright-local we need to update the project node version ">=18" so it's compatible with playwright 1.56.1

Mobile tests are now running successfully on GitHub actions

[curlyfriesplease]

I think the bump to Node 20 didn't work because of the versions of cache/setup-node/checkout, Seb's PR here https://github.com/comicrelief/serverless-payments/pull/807 makes this point. Sorry I didn't know that when I left the message on slack yesterday!
Looks like you can bring the versions up to their latest (checkout v6, setup-node v6, cache v5) if you want to, like how he has there.

[curlyfriesplease]

Merge if you've got a lot on. But if you have the time to see if those latest versions work, please do so and we can drop the reliance on older unsupported node versions

[KrupaPammi]

I tried updating the GitHub Actions versions to the latest, but both Playwright jobs are now failing. Wondering if I’ve missed anything from Seb’s PR? Otherwise, I can revert this for now and update these actions and node once we have a plan to do so?

[KrupaPammi]

@curlyfriesplease could you re-review this please?

[curlyfriesplease]

Thanks for trying it. In that case please do go ahead with this working set of versions.