Ignore release train workflow files in Dependabot (#3263)

Dependabot was opening GitHub Actions update PRs for release train
workflow files that are auto-generated and not intended for direct
dependency maintenance. Adding exclude-paths prevents noisy update
churn for those generated workflows.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: ryanjbaxter

.github/dependabot.yml

@@ -3,6 +3,9 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     target-branch: "main"
+    exclude-paths:
+      - ".github/workflows/release-train-test.yml"
+      - ".github/workflows/release-train-build.yml"
     schedule:
       interval: "weekly"
   - package-ecosystem: maven