Remove groups/communities support from Synapse

[clokep]

The groups/communities feature in Synapse is unspecced (defined in MSC1513, kind of, but is abandoned) -- spaces (MSC1772 and friends).

There are still some useful features driven by Groups (namely flair) so it is unclear when this will happen. Note that Element Web has the capability to convert a Group -> Space.

A rough plan of how we might want to do this is:

• Add an experimental config flag for groups that guards the unstable endpoints for groups.
• Add a deprecation notice to our release and upgrade notes. (Should probably be done at the same time as 1.)
• Default the above flag to false to disable those endpoints at the end of the deprecation period. The configuration flag should emit a warning if it is enabled.
• (After some time...) Remove the groups code.
  • Endpoints
  • Handlers
  • Update the worker documentation
  • RelatedGroups constant
• (After some time...) Bump the schema and remove the database tables for groups.

Related to #8334.

[clokep]

I'm not really sure if there's much benefit to step 3 -- we might want to just jump over it and do step 4 at the end of the deprecation period.