[ObjC] Tweaks to the Objective-C github actions.

- General naming up to "Objective-C" that some linters want, so just
  use it every where for consistence.
- Add a bazel macOS config
  - Includes the conformance test, so drop it from the Xcode flows.
- Tweak the naming on the Xcode flows so they are a little more clear.
- CocoaPods flows:
  - Give them (hopefully) clear naming
  - Use the same `image` as the Xcode tests
  - Pin the Xcode version to match the Xcode tests

PiperOrigin-RevId: 508156130