macos-13-xlarge to macos-15-xlarge

Summary:
Internal

https://fburl.com/z8uwzn57

> This is a scheduled macos-13-xlarge brownout. The macOS-13 based runner images are being deprecated. For more details, see actions/runner-images#13046.

> Workflows using the macos-13 image label should be updated to macos-latest, macos-14, or macos-15.


Using macos-15 here

Differential Revision: D86784053

Author: AbishekS

.github/workflows/python-unittests.yaml

@@ -14,7 +14,7 @@ jobs:
         platform: [linux.24_04.4x]
         include:
           - python-version: 3.12
-            platform: macos-13-xlarge
+            platform: macos-15-xlarge
       fail-fast: false
     env:
       OS: ${{ matrix.platform }}