ci: setup Dependabot for the Go modules

.github/dependabot.yaml

@@ -1,5 +1,27 @@
+---
 version: 2
 updates:
+  -
+    package-ecosystem: gomod
+    directory: /
+    schedule:
+      interval: weekly
+    commit-message:
+      prefix: chore
+  -
+    package-ecosystem: gomod
+    directory: /caddy
+    schedule:
+      interval: weekly
+    commit-message:
+      prefix: chore(caddy)
+    # These packages must be in sync with versions
+    # used by github.com/caddyserver/caddy/v2
+    ignore:
+      -
+        dependency-name: github.com/google/cel-go
+      -
+        dependency-name: github.com/quic-go/*
   -
     package-ecosystem: github-actions
     directory: /