- Adjust latest supported macOS logic for e.g. Big Sur 11.1.
- Updated latest supported version in docs to Mojave

Fixes https://github.com/Homebrew/brew/issues/9211

- Suppress (some more) warnings when doing `brew install --quiet`
- Clarify `man brew` output that we don't suppress all warnings
  for all commands with `--quiet`

While I was doing this I noticed references to the (soon to be
deprecated) `brew switch` so:
- remove these references in `install` output
- remove a reference in the documentation
- add a comment to remind me to deprecate `brew diy`, too

Fixes #9179

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.16.0 to 3.17.0.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.16.0...v3.17.0

)

Signed-off-by: dependabot[bot] <support@github.com>

- 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

Fix `-1` and other flags so they're handled correctly with casks. Use
the "right" exceptions for declaring invalid combinations and change
their parent class so that `--help` is printed nicely too.

Fixes #9033

The way we currently handle @-versioned formulae linking is pretty
labourius:
- it requires extensive use of `link_overwrite` to avoid the `link`
  stage failing on certain install/upgrade scenarios
- we teach people to use `brew link --force` whenever they wish to
  link a versioned formulae when it's pretty obvious what's expected
  in that situation

Instead, let's:
- automatically unlink other versioned formulae when linking a
  versioned formula (either through `brew link` or `install`/`upgrade`
  /`reinstall`)
- notify the user what we've done (with the same messaging as if
  they had run `brew link` manually)

Let's slow down a little on enhancement PRs to give other maintainers
time to give their feedback.

This is mostly self-directed criticism.

- I don't necessarily agree with this, but it's AP style that words with
  four or more letters should start with a capital when in titles.

- I added these all the way back in December 2019 in #6826, but only
  today realised that things have been failing unnoticeably because
  "warning" level exits with status code 0, so doesn't fail CI.
- Upgrading to "error" level will enable us to see the style errors in
  our docs and fix them faster.
- My editor also auto-formatted the YAML to have double quotes instead
  of single.