Bump dependabot open PR limit (apache#1597)

Bump dependabot open PR limit to 50, similar to
[iceberg-rust/apache#921](apache/iceberg-rust#921)

Author: kevinjqliu

.github/dependabot.yml

@@ -23,7 +23,7 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
-    open-pull-requests-limit: 5
+    open-pull-requests-limit: 50
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: