Deprecate @libp2p/mplex tracking issue

[maschad]

As a part of the wider initiative to deprecate mplex we need to:

• Add a deprecation notice the @libp2p/mplex README.
• Add a deprecation notice to the Multiplexing section of the GETTING_STARTED.
• Deprecate the@libp2p/mplex module on npm
• Remove all tests using mplex
• Remove all references in the docs to mplex

[achingbrain]

Remove all tests using mplex

The missing piece here is to replace mplex with yamux where a muxer is required by the test.

The thing about mplex is that it's in this repo which means when we next release a major of @libp2p/interfaces we don't need to solve the chicken and egg problem of the tests here not passing (and as such the release not happening) until yamux upgrades, which can't happen until the tests here pass (and the release happening).

I think mplex will still need to be used in tests here for the foreseeable future.

[maschad]

The missing piece here is to replace mplex with yamux where a muxer is required by the test.

I should have articulated it this way, but post-#1578 this should be the case, so our tests are currently testing against yamux to my understanding. Therefore, it should be safe to remove mplex from those tests.

[achingbrain]

Mplex is marked deprecated so there's not much else to do here.

It's still in the repo and being tested/updated as it's very low-maintenance to do so. If it becomes high-maintenance it'll be removed but that's not happened yet.