Add a Dependabot config to auto-update GitHub action versions (#2847)

Author: kurtmckee

.github/dependabot.yml

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

CHANGES

@@ -11,6 +11,7 @@
     * asyncio: Fix memory leak caused by hiredis (#2693)
     * Allow data to drain from async PythonParser when reading during a disconnect()
     * Use asyncio.timeout() instead of async_timeout.timeout() for python >= 3.11 (#2602)
+    * Add a Dependabot configuration to auto-update GitHub action versions.
     * Add test and fix async HiredisParser when reading during a disconnect() (#2349)
     * Use hiredis-py pack_command if available.
     * Support `.unlink()` in ClusterPipeline