Add Dependabot config with 5-day cooldown

Mirrors the cli/cli Dependabot configuration:

- gomod, daily, ignore major-version updates
- github-actions, daily

Adds a 5-day `cooldown.default-days` to both ecosystems so newly
released versions have time to bake before Dependabot opens a PR for
them.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: williammartin

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+updates:
+- package-ecosystem: gomod
+  directory: "/"
+  schedule:
+    interval: "daily"
+  cooldown:
+    default-days: 5
+  ignore:
+    - dependency-name: "*"
+      update-types:
+        - version-update:semver-major
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+    interval: "daily"
+  cooldown:
+    default-days: 5