- Output `brew doctor` and `brew install` messages noting this configuration is (currently) unsupported and encourage use of Rosetta instead
- Output Rosetta 2 usage in `brew config` on ARM (whether in Rosetta 2 or not)
- Check the architecture of (newly installed) dependencies and ensure they are using the correct architecture.
- Don't allow installing macOS Intel Homebrew in macOS ARM Homebrew default prefix (and vice versa
- Actually write out the architecture of dependencies to the tab rather than generating and throwing them away
- Set and document the expected default prefix for macOS Intel Homebrew, macOS ARM Homebrew (`/opt/homebrew`) and Homebrew on Linux

While we're here:
- Don't say Big Sur is a prerelease version but still make it clear we
  don't support it (yet).
- Don't reference non-existent IRC channel

Raised in #9036.

Python emits the version on either stderr or stdout based on major
version, so just to be safe let's use popen_read and combine the
output streams.

Reuse code from `brew doctor` that checks and produces a fatal error
and from `install.rb` that runs it.

Closes #8646

Until these feature flags are fully enabled/removed these will result
in uncommitted gems (which will block CI).

Also look in Homebrew/homebrew-core for uncommitted files (in some ways
we care about uncommitted stuff there more than Homebrew/brew). Also,
list all the modified files even when on non-CI to make it easier for
users to figure out their own issues and check the contents of the
whole repos rather than just a subdirectory.

In general this is a cleanup that should have been done long ago (when
we split Homebrew/brew and Homebrew/homebrew-core).

Fixes #6909.