Added 'Requires Plugins: wordpress-seo' header

[josevarghese]

Requires Plugins: header helps to show a message to activate the Yoast SEO plugin first; in case if it was not enabled.

Summary

This PR can be summarized in the following changelog entry:

• The Requires Plugins: header helps to show a message that the Yoast SEO free plugin is required to activate the Yoast Test Helper plugin on the Plugins page and will also show a message if someone tries to deactivate the Yoast SEO plugin without deactivating the Yoast Test Helper plugin.

Relevant technical choices:

Milestone

• I've attached the next release's milestone to this pull request.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Condition: 1

• Deactivate the Yoast SEO plugin, and try to install and activate the Yoast Test Helper
• It shows a message that the Yoast SEO plugin needs to be activated

Condition 2:

• Ensure that both Yoast SEO and Yoast Test Helper plugins are active
• Try to deactivate the Yoast SEO plugin, then you can notice a message that you need to deactivate the Yoast Test Helper plugin first to deactivate the Yoast SEO

Fixes #