CI: Bump to Ubuntu 22.04 in the GMT Legacy Tests workflow

Author: seisman

.github/workflows/ci_tests_legacy.yaml

@@ -34,7 +34,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, macos-13, windows-2019]
+        os: [ubuntu-22.04, macos-13, windows-2019]
         gmt_version: ['6.4']
     timeout-minutes: 30
     defaults: