We were previously only looking at the `cellar` value when pouring
bottles and ignoring the `prefix` and (implicit) `repository`.

Actually look at these values and set the defaults for each platform.

Also, when we're relocating to create or pour bottles when `prefix`
and `repository` are equal then skip relocating the `repository` and
always use references to the `prefix` instead.

Fixes #9453

- 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