dependabot: group together common deps (#950)

Group together deps that are often updated together.

- https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups

.github/dependabot.yml

@@ -9,3 +9,14 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    groups:
+      golang-x-dependencies:
+        patterns:
+          - "golang.org/x/*"
+      zx2c4-dependencies:
+        patterns:
+          - "golang.zx2c4.com/*"
+      protobuf-dependencies:
+        patterns:
+          - "github.com/golang/protobuf"
+          - "google.golang.org/protobuf"