ci: track GitHub Actions updates on dev-v3 via Dependabot

Dependabot reads its config only from the default branch, and the
github-actions ecosystem was configured for main only. As a result,
Action version bumps never flowed to dev-v3, leaving its workflows
(e.g. govulncheck-action) frozen and drifting from main.

Add a github-actions update block targeting dev-v3 so Action bumps are
opened there too, matching the existing gomod/dev-v3 arrangement.

Signed-off-by: Terry Howe <terrylhowe@gmail.com>

.github/dependabot.yml

@@ -37,3 +37,9 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+  - # Keep dev-v3 GitHub Actions up to date, while Helm v3 is within support
+    package-ecosystem: "github-actions"
+    target-branch: "dev-v3"
+    directory: "/"
+    schedule:
+      interval: "daily"