Use macos-12 GHA runners instead of macos-latest

[mjameswh]

What was changed

• In CI, use macos-12 GHA runners instead of macos-latest, as `macos

Why?

• GitHub Action’s runner macos-latest has just been updated to point to macos-14, which are M1-based machines, rather than macos-12, which were x64-based machines.

This is a breaking change at this time, as there is no binary distribution of protoc 3.x for that architecture.

```
Run arduino/setup-protoc@v1
Getting protoc version: v3.20.3
Downloading archive: https://github.com/protocolbuffers/protobuf/releases/download/v3.20.3/protoc-3.20.3-osx-aarch_64.zip
Error: Error: Failed to download version v3.20.3: Error, Unexpected HTTP response: 404 - 404
```

[cretz]

Thanks!

There is an open issue at #210, and we should support both Intel and ARM mac (same as temporalio/sdk-python#506) and test both and build packages on both. Right now we only test on one and use cross-compilation for build with no smoke test for this version.

But this is a good stopgap to get over whatever just changed. Also there's a build-binaries GH thing that has to be fixed too probably. Unsure if I can change branch protection on this yet, will see.

[mjameswh]

@cretz I modified required checks to include macos-12 instead of macos-latest. We'll need to update that again once we implement the correct solution.