Add dependabot configuration for npm dependencies (#5378)

Motivation:

The following [PR](#5377) attempts
to update the `package-lock.json` file only.
I've read through the following documents, and believe that specifying
the versioning strategy will help update the actual `package.json`
dependencies

Modifications:

- Added a versioning strategy for dependabot
- Specified the interval to monthly so that it is better in-sync with
our release schedule and we don't receive multiple update pull requests

Result:

- Improved dependabot pull requests

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.github/dependabot.yml

@@ -29,3 +29,9 @@ updates:
           time: "10:00"
           # Use Korea Standard Time (UTC +09:00)
           timezone: "Asia/Seoul"
+
+    - package-ecosystem: "npm"
+      directory: "/"
+      schedule:
+        interval: "monthly"
+        versioning-strategy: increase