Update dependabot.yml

Shortened prefix from "github_actions" to "gh_actions" in order to meet the 15 character max length requirement
diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index e24ee23..02bfe34 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -291,7 +291,7 @@
   - package-ecosystem: "github-actions"
     directory: "/"
     commit-message:
-      prefix: "[github_actions]"
+      prefix: "[gh_actions]"
     schedule:
       interval: "daily"
     open-pull-requests-limit: 10