chore: fix latest compile TS and Node 20 workflow failures

[JamesHenry]

Description:

In PR #7511 I discovered that Node 18 tests are working fine, but it looks like the latest Node 20 (20.18.0) which the Node 20 job is configured to use, has actually implemented FinalizationRegistry which was added to the tests 4 years ago (!) but has not actually been in use because of it not being present globally in Node until now.

This is obviously a very surprising discovery that this test code has not actually ever been executed in the last 4 years, and in reading the MDN entry for FinalizationRegistry (a thing I was not familiar with at all), it seems like something we'd want to avoid in deterministic tests across multiple environments.

Therefore I replaced its usage entirely with a WeakRef which should hopefully cover the desired logic on Node v20.

(https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry)

The PR then ultimately also fixes the long standing TS compilation issues with latest TS.

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[JamesHenry]

Ha, so this did indeed fix v20, but has caused v18 to fail... I will see if I can repro locally. Perhaps it just needs a few retries on the GC check...

[JamesHenry]

It turns out global.gc does not exist by default in Node 18. We could either:

• skip the tests in Node 18
• always run node explicitly for the mocha tests and pass it the --expose-gc flag which sets global.gc. Again, it's set by default in Node 20.

I decided to go with the latter, but let me know if you would prefer the former.

[JamesHenry]

NOTE: This will still be blocked until the type error on latest TS is resolved

Fixed in commit https://github.com/ReactiveX/rxjs/pull/7512/commits/72884e22b93aab8e58e12e2e8b90d24748f469c0