test/github_runner_matrix: prevent dropping supported macOS versions from CI

We should bump `HOMEBREW_MACOS_OLDEST_SUPPORTED` first and make sure
that bump goes out into a release tag before dropping CI in
Homebrew/core. Otherwise, we end up with users getting errors about
missing bottles. See, for example, Homebrew/homebrew-core#240857.

We've made it very hard for users on OS versions newer than
`HOMEBREW_MACOS_OLDEST_SUPPORTED` to *not* use bottles (because only
bottles are supported), but that design assumes we have been building
bottles for them. We should therefore make sure that users we stop
building bottles for are always on a version older than
`HOMEBREW_MACOS_OLDEST_SUPPORTED`.

Author: carlocab

Library/Homebrew/test/github_runner_matrix_spec.rb

@@ -33,6 +33,13 @@
     setup_test_runner_formula("testball-depender-newest", ["testball", { macos: symbol }])
   end
 
+  describe "OLDEST_HOMEBREW_CORE_MACOS_RUNNER" do
+    it "is not newer than HOMEBREW_MACOS_OLDEST_SUPPORTED" do
+      oldest_macos_runner = MacOSVersion.from_symbol(described_class::OLDEST_HOMEBREW_CORE_MACOS_RUNNER)
+      expect(oldest_macos_runner).to be <= HOMEBREW_MACOS_OLDEST_SUPPORTED
+    end
+  end
+
   describe "#active_runner_specs_hash" do
     it "returns an object that responds to `#to_json`" do
       expect(