Fix bit rot in GitHub workflows

[graebm]

Issue:

1. The osx CI broke, because GitHub's macos-latest runner changed from being x64 to being arm64. macos arm64 doesn't support older Ruby versions.

2. Some CI runs broke due to installing the latest bundler, which no longer works with Ruby 2.x

Description of changes:

1. Always use explicit runner versions, so CI doesn't unexpectedly break on us as frequently. I.e. macos-latest -> macos-14
2. Now that arm64 macos runners are available, actually run tests on them (instead of just a cross-compile)
3. Don't manually install bundler. The latest version doesn't work with Ruby 2.x. The setup-ruby action was already handling installing a compatible version

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.