Configure Dependabot (#525)

Some of the GitHub Actions are outdated, leading to
[warnings](https://github.com/hazelcast/hz-docs/actions/runs/26168054589?pr=2195):
> Node.js 20 actions are deprecated. The following actions are running
on Node.js 20 and may not work as expected: actions/checkout@v4,
actions/setup-node@v4. Actions will be forced to run with Node.js 24 by
default starting June 2nd, 2026. Node.js 20 will be removed from the
runner on September 16th, 2026

Author: JackPGreen

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"