.github: add dependabot config to update go.mod and github actions.

Signed-off-by: David Anderson <danderson@tailscale.com>

.github/dependabot.yml

@@ -0,0 +1,16 @@
+# Documentation for this file can be found at:
+# https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/configuration-options-for-dependency-updates
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    commit-message:
+      prefix: "go.mod:"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    commit-message:
+      prefix: ".github:"