Add WP version check workflow

[mi5t4n]

Description

Whenever a new version of WP is released, we are required to update the Tested up to information in the readme.txt file. This PR adds a workflow which is scheduled to run at midnight on every Monday.

Technical Details

1. Uses skaut/wordpress-version-checker GitHub action to check for the discrepancy between the latest WP version and the WP version in the readme.txt
2. The workflow is scheduled to run at midnight on every Monday.
3. Raises an GitHub issue suggesting we need to update the Tested up to in the readme.txt on the repository.

Improvement

We can add automatically assign the issues raised by the workflow to a developer, by adding the following code in the .wordpress-version-checker.json.

{
    "assignees": ["alice", "bob"]
}

Reference:

https://github.com/10up/simple-local-avatars/blob/develop/.github/workflows/wordpress-version-checker.yml

Fixes/Covers issue

Fixes #172