Revert "Bump google-cloud-pubsub to 2.9.0 (to bump grpcio)"

[frenck]

Reverts #63493

https://github.com/home-assistant/core/runs/4719615426?check_suite_focus=true

Make our wheels builds fail.

39-i386-linux-gnu.so -lpthread -Wl,-wrap,memcpy -static-libgcc -lexecinfo
  /usr/lib/gcc/i586-alpine-linux-musl/10.3.1/../../../../i586-alpine-linux-musl/bin/ld: cannot find -lpthread -Wl,-wrap,memcpy -static-libgcc -lexecinfo
  collect2: error: ld returned 1 exit status
  Traceback (most recent call last):
    File "/usr/local/lib/python3.9/distutils/unixccompiler.py", line 204, in link
      self.spawn(linker + ld_args)
    File "/tmp/pip-wheel-z3akzlva/grpcio/src/python/grpcio/_spawn_patch.py", line 54, in _commandfile_spawn
      _classic_spawn(self, command)
    File "/usr/local/lib/python3.9/distutils/ccompiler.py", line 910, in spawn
      spawn(cmd, dry_run=self.dry_run)
    File "/usr/local/lib/python3.9/distutils/spawn.py", line 91, in spawn
      raise DistutilsExecError(

Discussed this with @allenporter on Discord as the best action to take for now.