Minor main.yml Improvements

### Housekeeping

- [X] I am a maintainer of dbt-core

### Short description

Make `main.yml` more efficient by pulling out the code quality tests and adjusting the scheduled testing.

### Acceptance criteria

- Pull out the code quality checks to a separate workflow that always runs
- New code quality check should be a required check
- Remaining jobs in `main.yml` (unit/functional/builds) should only run when necesary
- exclude when the only files changes are in `.github` or `.changes`. Possibly others?
- Update the [scheduled branch testing](https://github.com/dbt-labs/dbt-core/blob/main/.github/workflows/release-branch-tests.yml) to also run the code quality checks now that they're in a separate workflow
- There are 2 options to do this
- backport the workflow split all the way back to 1.0
- add logic to run different workflow files based on the branch that's running. This would require changes to teh workflow in `actions` so may not be worth the lift.

### Suggested Tests

This is just tesating

### Impact to Other Teams

None

### Will backports be required?

Possibly. But no releases would be related to the backports. Depends on how we handle scheduled testing.

### Context

Determine if the `main.yml` workflow is run anywhere else that we would also need to run the code quality checks.

For example, we run the main.yml workflow in dbt-common but since it's just testing functionality with dbt-common, it's okay to skip code-quality checks there so it requires no updates.