Upgrade to GitHub-native Dependabot (#288)

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

packages/ripple-keypairs/.github/dependabot.yml

@@ -0,0 +1,25 @@
+version: 2
+updates:
+- package-ecosystem: npm
+  directory: "/"
+  schedule:
+    interval: weekly
+    time: "15:00"
+  open-pull-requests-limit: 20
+  ignore:
+  - dependency-name: "@types/node"
+    versions:
+    - 14.14.22
+    - 14.14.25
+    - 14.14.28
+    - 14.14.32
+    - 14.14.34
+    - 14.14.35
+    - 14.14.37
+  - dependency-name: mocha
+    versions:
+    - 8.3.1
+    - 8.3.2
+  - dependency-name: typescript
+    versions:
+    - 4.2.2