test_observer requires macOS 13.0

When updating rules_swift on swift-syntax I noticed that test_observer gets automatically added and requires macOS 13.0 based on some of the APIs it's using. swift-syntax currently pins in SwiftPM to 10.15, so I was trying to keep that matched in bazel.

Seems to me like we can do a few things:

1. Force a minimum macOS version supported by rules_swift
2. Change test_observer to support older macOS versions
3. Disable test_obeserver if we're targeting something lower than 13.0

thoughts?

Start with test_observer and swift-syntax's SwiftPM pin to macOS 10.15, then identify which APIs impose the macOS 13.0 requirement and how Bazel's target version is selected. Done requires an agreed compatibility approach among raising the minimum, changing test_observer, or disabling it for older targets, followed by validation against the supported macOS versions.

Written by the indexing model from the issue text.