[9.0] Revert "Dependency: Update to Prometheus 0.300.* (#36873)"

[cmacknz]

This reverts commit af6b17e.

Removes the update to Promtheus 0.300 from the v0.120.0 release of the upstream collector contrib repository.

This will be merged to a 9.0 branch in our fork (and an 8.18 branch created from the same point once this merges). This will let Elastic Agent's 9.0 and 8.18 branches include all other fixes from the v0.120.0 collector contrib release except this one.

Once this merges, I will create a tag that I intend to name v0.120.2-elastic to indicate this release does not exist upstream yet and is elastic specific so that we can more clearly see what we depend on upstream.

I have proven this fixes our dependency conflict in elastic/elastic-agent#7411 which replaces the upstream dependency with this branch in my fork.

For whatever reason, simply running make on my MacOS machine fails with what seems like unrelated errors, however make -C in each directory touched by af6b17e succeeds without issue.

The biggest thing I need help with is verifying I have not inadvertently broken anything without noticing it.

[swiatekm]

I ran the full unit test suite locally, as well as integration tests for Prometheus receiver. Everything passed, minus some tests which depend on the Github Actions runner environment (host metrics tests assume you have swap enabled on linux/amd64 for example). We should be safe merging this.