Run GMT Dev Tests on Monday, Wednesday and Friday only (#1922)

* Schedule GMT Dev Tests on Monday, Wednesday and Friday only
* Update doc/maintenance.md to mention Mon/Wed/Fri cron schedule

Author: weiji14

.github/workflows/ci_tests_dev.yaml

@@ -17,9 +17,9 @@ on:
       - '.gitignore'
   repository_dispatch:
     types: [test-gmt-dev-command]
-  # Schedule daily tests
+  # Schedule tests on Monday/Wednesday/Friday
   schedule:
-    - cron: '0 0 * * *'
+    - cron: '0 0 * * 1,3,5'
 
 jobs:
   test_gmt_dev:

doc/maintenance.md

@@ -108,8 +108,8 @@ There are 11 configuration files located in `.github/workflows`:
 4. `ci_tests_dev.yaml` (GMT Dev Tests on Linux/macOS/Windows).
 
    This is triggered when a PR is marked as "ready for review", or using the
-   slash command `/test-gmt-dev`. It is also scheduled to run daily on the
-   *main* branch.
+   slash command `/test-gmt-dev`. It is also scheduled to run on Monday,
+   Wednesday and Friday on the *main* branch.
 
 5. `cache_data.yaml` (Caches GMT remote data files needed for GitHub Actions CI)