Pin Jetty version to 9.4.x (#11453)

Major version bumps in jetty are too scary for now. So let's keep up to date with the latest 9.4.x
diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index 717d6a0..7640594 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -9,3 +9,6 @@
       # pin ZooKeeper dependencies to 3.5.x
       - dependency-name: "org.apache.zookeeper"
         versions: "[3.6,)"
+      # pin Jetty dependencies to 9.4.x
+      - dependency-name: "org.eclipse.jetty"
+        versions: "[9.5,)"