chore: add cooldown to dep upgrades

Because:

* We want to give dependency upgrades some bake time

This commit:

* Adds a cooldown to dependabot

Author: clouserw

.github/dependabot.yml

@@ -4,7 +4,11 @@ updates:
     directory: '/'
     schedule:
       interval: 'daily'
-    open-pull-requests-limit: 6
+    cooldown:
+      semver-major-days: 30
+      semver-minor-days: 14
+      semver-patch-days: 7
+    open-pull-requests-limit: 2
     labels:
       - 'maintenance'
       - 'dependencies'
@@ -27,7 +31,11 @@ updates:
     directory: '/'
     schedule:
       interval: daily
-    open-pull-requests-limit: 6
+    cooldown:
+      semver-major-days: 30
+      semver-minor-days: 14
+      semver-patch-days: 7
+    open-pull-requests-limit: 2
     reviewers:
       - '@mozilla/fxa-devs'
     labels: