ci: run tests against Firebase emulator

[thatfiredev]

This PR should upgrade our CI to:

• use versions 20 and 22 of NodeJS
• use the latest versions of GH Actions
• run the automated tests against the emulator instead of an actual Firebase project (it used to be geofire-gh-tests)
• use placeholders for the Firebase configuration in the examples

I went ahead and changed the rules in that project to not allow any writes or reads. (internal bug: b/324238753)

[coveralls]

coverage: 89.041% (-3.2%) from 92.237%
when pulling d1ec694 on rpf-run-tests-in-emulator
into fb22381 on master.