github: dependabot: configure cooldown time for updates

This will delay non security dependabot updates to packages,
giving automated tools and researchers more time to catch updates
with malicious intent, thus reducing the supply chain security risks.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>

.github/dependabot.yml

@@ -5,6 +5,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 7
     commit-message:
       prefix: "ci: github: "
     labels: []
@@ -17,6 +19,8 @@ updates:
     directory: "/doc"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 7
     commit-message:
       prefix: "ci: doc: "
     labels: []