Use main version of reusable WF in merge-dependabot-pr.yml

The `spring-io/spring-github-workflows/.github/workflows/spring-merge-dependabot-pr.yml` behaviour has been changed.
So, this change is to test it for the next Dependabot updates cycle.

Author: artembilan

.github/workflows/merge-dependabot-pr.yml

@@ -12,7 +12,7 @@ jobs:
   merge-dependabot-pr:
     permissions: write-all
 
-    uses: spring-io/spring-github-workflows/.github/workflows/spring-merge-dependabot-pr.yml@v5
+    uses: spring-io/spring-github-workflows/.github/workflows/spring-merge-dependabot-pr.yml@main
     with:
       mergeArguments: --auto --squash
       autoMergeSnapshots: true