CI: Use macOS 15 in the 'GMT Dev Tests' workflow (#3485)

Author: seisman

.github/workflows/ci_tests_dev.yaml

@@ -36,7 +36,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-24.04, macos-14, windows-2022]
+        os: [ubuntu-24.04, macos-15, windows-2022]
         gmt_git_ref: [master]
     timeout-minutes: 30
     defaults: