Throttle dependabot (mobilecoinfoundation#2874)

The motivation for this is, dependabot may under various circumstances
decide to rebase and force push all its open branchs. With the current
configuration, that can cause 50 PRs to trigger CI all at once, which
effectively DOS'es the engineers from using CI.

This change will cut down the number of open dependabot PRs at any
point in time by about a factor of 5, so these force push events
will cause much less load on the cluster and engineers will not be
forced to manually cancel dependabot CI runs to get their work done.

.github/dependabot.yml

@@ -6,7 +6,7 @@ updates:
     interval: daily
     time: "22:00"
     timezone: "America/Los_Angeles"
-  open-pull-requests-limit: 25
+  open-pull-requests-limit: 5
   labels:
     - "blocks-v4.0.0"
     - "dependencies"
@@ -17,7 +17,7 @@ updates:
     interval: daily
     time: "23:00"
     timezone: "America/Los_Angeles"
-  open-pull-requests-limit: 5
+  open-pull-requests-limit: 2
   labels:
     - "blocks-v4.0.0"
     - "dependencies"
@@ -28,7 +28,7 @@ updates:
     interval: daily
     time: "00:00"
     timezone: "America/Los_Angeles"
-  open-pull-requests-limit: 5
+  open-pull-requests-limit: 2
   labels:
     - "blocks-v4.0.0"
     - "dependencies"
@@ -39,7 +39,7 @@ updates:
     interval: daily
     time: "01:00"
     timezone: "America/Los_Angeles"
-  open-pull-requests-limit: 5
+  open-pull-requests-limit: 2
   labels:
     - "blocks-v4.0.0"
     - "dependencies"
@@ -50,7 +50,7 @@ updates:
     interval: daily
     time: "02:00"
     timezone: "America/Los_Angeles"
-  open-pull-requests-limit: 5
+  open-pull-requests-limit: 2
   labels:
     - "blocks-v4.0.0"
     - "dependencies"
@@ -61,4 +61,4 @@ updates:
     interval: daily
     time: "03:00"
     timezone: "America/Los_Angeles"
-  open-pull-requests-limit: 10
+  open-pull-requests-limit: 2