Cask: only quarantine working symlinks

Fixes Homebrew/homebrew-cask#70796

Move condition to nested `if` statement.

Fix `Dmg` detection.

Bump Rubocop's `TargetRubyVersion` to 2.6 and fix all offenses

upgrade: reject formulae checked in earlier levels

- Only try to call `.match?` on strings that aren't nil.

Fix `Dmg` being detected as `Tar`.

- To have the audit commands continue to work, and the tests passing
  (the previous error was [1]), everywhere we're trying to
  `.match?(text)`, instead do `.match?(text.to_s)`.

[1] 5) Homebrew::FormulaAuditor#audit_file __END__ but no DATA
    Failure/Error: fa.audit_file

    TypeError:
      no implicit conversion of Homebrew::FormulaText into String
    # ./dev-cmd/audit.rb:267:in `match?'
    # ./dev-cmd/audit.rb:267:in `audit_file'
    # ./test/dev-cmd/audit_spec.rb:137:in `block (3 levels) in <module:Homebrew>'

- The final two `brew style` offenses were for Metrics/LineLength where
  parsing the GitHub diff URL as a regex took 119/118 lines.
- This disables the cop for each of those long, problematic lines, and
  re-enables them straight afterwards, else they'd apply to the whole file
  from the first occurrence onwards, which we probably don't want?
- I wondered about increasing the LineLength limit to 119 or 120, but
  it appears to have been set for a justifiable reason in 80d75bc0.

- These didn't appear in the first pass as the other offense fixes caused them.

- This also required auto-fixes for Layout/EmptyLinesAroundBlockBody and
  Layout/InconsistentIndentation once the auto-fixer had got rid of the
  "redundant begin"s.

- This was set to 2.3 because Homebrew was running Ruby 2.3.7. Since
  https://github.com/Homebrew/brew/pull/6556, we're running 2.6.3. With
  this change, Rubocop will check against modern Ruby syntax.

bash completion example should not call brew if brew doesn't exist

remove /usr/bin/xattr unsupported -s option in macOS 10.15

Remove uneeded warning when searching for a cask migrated from formula

Docs: Add cask instructions to tap docs

Show correct message when invalid arguments are passed to brew

Expose `uses_from_macos` list in formula API

Fix frozen string bug