CI: Bump to macOS 14 in the GMT Legacy Tests workflow (#3996)

Author: seisman

.github/workflows/ci_tests_legacy.yaml

@@ -36,7 +36,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-22.04, ubuntu-22.04-arm, macos-13, windows-2022]
+        os: [ubuntu-22.04, ubuntu-22.04-arm, macos-14, windows-2022]
         gmt_version: ['6.4']
     timeout-minutes: 30
     defaults: