Decision: allow Hadoop Yarn to be phased out due to missing maintenance capacity?

I'd like to propose we stop attempting to support Hadoop Yarn in this project by:
- documenting it is being phased out unless maintenance effort beyond current capacity arrives, specifically maintenance around:
- maintenance of the dependency [`skein`](https://github.com/jcrist/skein) (https://github.com/jcrist/skein/issues/255)
- making a new CI testing image not based on CentOS 7, which no longer builds (https://github.com/dask/dask-gateway/issues/837)
- allowing it to be cut away from this project over time when we encounter issues maintaining functional tests etc.

As a first step, I'd like to merge https://github.com/conda-forge/dask-gateway-feedstock/pull/43 and drop the conda-forge feedstock's output `dask-gateway-server-yarn` that makes the entire feedstock not get upgraded to emit Python 3.13 outputs because its dependency `skein` isn't updated for Python 3.13.

_**I'm looking for a conclusion in this issue around whether it is okay to proceed with phasing out Hadoop Yarn as friction arises in maintenance while waiting for eventual maintenance capacity to arise.**_

/cc: @TomAugspurger @jacobtomlinson @martindurant @jcrist