Add workflow to cleanup caches

[BSFishy]

Description

Adds a workflow that cleans up old caches when a PR is merged. Follows the example in these docs: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries

Issues Resolved

Closes #211

Check List

• New functionality includes testing.
• New functionality has been documented.
• All tests pass
  • yarn lint
  • yarn test-unit
• Commits are signed per the DCO using --signoff

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[AMoo-Miki]

If we are not using the cache, let's just stop caching by removing it from here.

[BSFishy]

If we are not using the cache, let's just stop caching by removing it from here.

We're using the cache. Basically it looks like this:

PR raised
 |
 V
Build and test workflow started
 |
 V
Cache doesn't exist, run a new `yarn install`
 |
 V
Build and test with packages
 |
 V
Cache packages

Then, when a change is made in the PR:

Change is made
 |
 V
Build and test workflow started
 |
 V
Cache is found, use cached packages (theoretically faster than installing from scratch)
 |
 V
Build and test with packages

At no point do we actually remove these caches, so they're left as zombies after the PR is closed. This PR adds a workflow that will delete those zombie caches, saving space for caches for new PRs or for long-standing branches like main or 1.x.

[AMoo-Miki]

The caches expire after 7 days of inactivity. Us deleting the cache after a PR is merged would just make it unavailable to the next run. So the cache that was created in one PR which could have benefited another PR is removed after the first one is merged; the second one does not benefit.

I think we should just let GH handle the TTL of the caches: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy

[BSFishy]

We'll leave it up to Github then :)