Move macOS build to macos 15

Author: Steve Kirkland

.buildkite/unity.2020.full.yml

@@ -12,7 +12,7 @@ steps:
         key: build-macos-fixture-2020
         depends_on: build-artifacts
         agents:
-          queue: macos-14
+          queue: macos-15
         env:
           XCODE_VERSION: 15.3.0
           UNITY_PERFORMANCE_VERSION: *2020
@@ -102,6 +102,7 @@ steps:
         timeout_in_minutes: 10
         key: build-ios-fixture-2020
         depends_on: generate-fixture-project-2020
+        # macOS 14 used here due to build issues specifically for iOS/Unity 2020 in macOS 15
         agents:
           queue: macos-14-isolated
         env: