Update CI for latest macOS and Xcode

[luispadron]

Updates CI and the build script:

• Use macos-latest for latest macOS version and toolchain
• Replace package generate (which no longer exists) with a simple swift build
• Remove Bazel build support, bazel users should consume via https://registry.bazel.build/modules/rules_swift_package_manager.
• Remove Xcode job, the pod lib lint job still exists and continues to validate the podspec which is what we care about.
• Remove build.swift and instead update the github workflow to run the commands via Bash, this is a lot simpler and does the same in less code.

[dfed]

I'd recommend reverting this PR + taking another pass. Very down to modernize CI on this repo (and I'm happy to help!) but we really should not be removing all the tests from CI.

[dfed]

@luispadron I appreciate your trying to update CI here, but unless I'm missing something we've made a bit of an oops.

In this PR we've removed the instruction to run the demo app tests, and the demo app is where the tests for this project live.

You mention in the PR description that:

the pod lib lint job still exists and continues to validate the podspec which is what we care about

However, the podspec does not list any tests, and therefore I believe this PR actually removed any validation that Stagehand is behaving correctly. All we're testing in CI now is that Stagehand builds, which is pretty insufficient.

Moreover, by no longer building the sample app, it is now possible for the sample code shipped with Stagehand to not compile, which is less-than-ideal.

[dfed]

you can see here that we were only running tests on the Stagehand Demo App, which is no longer built or tested in CI.