Bump supported version to OTP-27, deprecate OTP-24

[ferd]

This is keeping in line with our support policy for versions.

Opened as a Draft PR until we can show Dialyzer is happy (✅) and all of CI works (❌).

• Elixir version support needed to be bumped to shelltests, but Elixir does not yet recognize 27.0 in its own versions
• the MacOS tests are failing because brew failed to update to 27.0 (erlang 27.0 Homebrew/homebrew-core#172171) (also see: https://elixirforum.com/t/erlang-27-not-available-in-homebrew/63897/5)
  • we could possibly move to asdf/kerl there, but this will make CI run incredibly longer.

[ferd]

@tsloughter what do you think about raising the baseline version and all checks, but still awaiting for OSX and shellcheck tests, since both depend on third party components we can't fix yet?

[tsloughter]

Fine with that.