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.