[libc++] Disable back-deployment CI on the release branch

As explained in the comment, we don't have macOS 10.15 builders anymore
in the fleet. Enabling those tests on the release branch would require
cherry-picking too many changes from `main`.

libcxx/utils/ci/buildkite-pipeline.yml

@@ -598,34 +598,37 @@ steps:
           limit: 2
     timeout_in_minutes: 120
 
-  # Test back-deployment to older Apple platforms
-  - label: "Apple back-deployment macosx10.9"
-    command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-10.9"
-    artifact_paths:
-      - "**/test-results.xml"
-    agents:
-      queue: "libcxx-builders"
-      os: "macos10.15" # TODO: For now, we're running the back-deployment tests for 10.9 on 10.15, because we don't have proper 10.9 machines
-      arch: "x86_64"
-    retry:
-      automatic:
-        - exit_status: -1  # Agent was lost
-          limit: 2
-    timeout_in_minutes: 120
-
-  - label: "Apple back-deployment macosx10.15"
-    command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-10.15"
-    artifact_paths:
-      - "**/test-results.xml"
-    agents:
-      queue: "libcxx-builders"
-      os: "macos10.15"
-      arch: "x86_64"
-    retry:
-      automatic:
-        - exit_status: -1  # Agent was lost
-          limit: 2
-    timeout_in_minutes: 120
+  # Those are disabled on the release/14.x branch because we don't have macOS 10.15 builders anymore
+  # in the fleet. Enabling those tests on the release branch would require cherry-picking too many
+  # changes from `main`.
+  # # Test back-deployment to older Apple platforms
+  # - label: "Apple back-deployment macosx10.9"
+  #   command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-10.9"
+  #   artifact_paths:
+  #     - "**/test-results.xml"
+  #   agents:
+  #     queue: "libcxx-builders"
+  #     os: "macos10.15" # TODO: For now, we're running the back-deployment tests for 10.9 on 10.15, because we don't have proper 10.9 machines
+  #     arch: "x86_64"
+  #   retry:
+  #     automatic:
+  #       - exit_status: -1  # Agent was lost
+  #         limit: 2
+  #   timeout_in_minutes: 120
+
+  # - label: "Apple back-deployment macosx10.15"
+  #   command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-10.15"
+  #   artifact_paths:
+  #     - "**/test-results.xml"
+  #   agents:
+  #     queue: "libcxx-builders"
+  #     os: "macos10.15"
+  #     arch: "x86_64"
+  #   retry:
+  #     automatic:
+  #       - exit_status: -1  # Agent was lost
+  #         limit: 2
+  #   timeout_in_minutes: 120
 
   - label: "AArch64"
     command: "libcxx/utils/ci/run-buildbot aarch64"