Update MacOS CI

[Gedochao]

This does the following:

• updates MacOS x86_64 workflows from macOS-13 to macOS-15-intel
• updates MacOS arm workflows from macOS-14 to macOS-15
• turns off MacOS x86_64 workflows running integration test suites for Scala 2.12, 2.13 and 3.3 LTS
  • we'll still run these suites for MacOS arm
  • the motivation is that MacOS x86_64 & arm runners from GitHub share a limited pool of available runners, which means each and every CI run waits for MacOS tests to finish running, effectively doubling, sometimes tripling CI times
    • this slows down development considerably, at times
  • given how rare it is for us to detect problems for these, I think it's sufficient to limit this platform to Scala 3 Next & Scala 3 Next RC (the cutting edge versions, most likely to detect anything)
  • this means the following jobs will have to be removed from requirements on the CI @tgodzik
    • native-macos-tests-2
    • native-macos-tests-3
    • native-macos-tests-4
• note that macOS-13 jobs are being deprecated, and the new macOS-15-intel runners will be available until August 2027
  • we will likely stop distributing MacOS x86_64 launchers when GitHub stops supporting them on the CI
  • more reading available at [macOS] The additional macOS 15 Sonoma Intel-based image will be available in GitHub Actions actions/runner-images#13045

[Gedochao]

I'm thinking a follow-up renaming -1--5 test suites into something human-comprehendable might be a good idea 😅
I'd rather do that in a separate PR, however