Includes:
* DEV: Remove external plugin linting (that's covered by CI in their repositories)
* DEV: Move lint stages to a separate workflow (partial de-`if`-ication of workflows)
* DEV: Run CI on `main` branch too
* DEV: Update postgres to 13
* DEV: Update redis to 6.x
Other changes:
* DEV: Remove matrix.os
* DEV: Remove env.BUILD_TYPE
* DEV: Remove env.TARGET
* DEV: Rename `build_types` config option to `build_type`
* DEV: Lowercase `target` and `build_type` names
* DEV: Rename `ci` to `tests`
* DEV: Rename `lint` to `linting`
* DEV: Lower the wizard qunit timeout (30 min -> 10)
* DEV: Ruby version is no longer configurable
* DEV: Run plugin tests only in the `plugins` target
* DEV: Use binstubs where applicable
* DEV: We don't open PRs to `tests-passed`
Moves the most important checks into a linter. It gets executed by Lefthook as well as the docker rake task and Github actions. Doing those checks in rspec takes too long and it produces errors when the discourse:test Docker image contains old, invalid locale files.
