Motivation:

* Improve test coverage

Modifications:

Enable macOS CI to be run on pull request commits and make the use of
the nightly runner pool for main.yml jobs explicit.

Result:

Improved test coverage.

Add static SDK CI workflow which runs on commits to PRs, merges to main
and daily on main.

---------

Co-authored-by: Cory Benfield <lukasa@apple.com>

Motivation:

Swift 6.1 has been released, we should add it to our CI coverage.

Modifications:

Add additional Swift 6.1 jobs where appropriate in main.yml,
pull_request.yml

Result:

Improved test coverage.

Motivation:

Swift 5.9 is no longer supported, we should bump the tools version and
remove it from our CI.

Modifications:

* Bump the Swift tools version to Swift 5.10
* Remove Swift 5.9 jobs where appropriate in main.yml, pull_request.yml

Result:

Code reflects our support window.