Update shivammathur/setup-php dependency to use @v2

[gudmdharalds]

Update dependency shivammathur/setup-php to use tag @v2; this eliminates manually updating the workflow to use the latest version number available.

This also resolves a few run-time warnings, see for example here.

TODO:

• [x] Update shivammathur/setup-php to use version @v2.
• [x] Check status of automated tests
• [x] Changelog entry (for VIP) [ #312 ]