chore(ci): add dependabot version updates for gomod and actions

Repo only had Dependabot security updates (reactive). Add weekly grouped
version updates for Go modules and GitHub Actions to keep dependencies
current and reduce CVE accumulation.

.github/dependabot.yml

@@ -0,0 +1,21 @@
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+    groups:
+      go-dependencies:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    groups:
+      github-actions:
+        patterns:
+          - "*"