Summary

This PR can be summarized in the following changelog entries:

• Removes all uses of meta keywords throughout the plugin.
• Removes the "noindex subpages" feature as Google has gotten much better at paginated series, and it's now actually detrimental to use.
• Because both features above are removed, removes the "Other" tab from the Titles & Metas settings screen.

Relevant technical choices:

• Deprecated the class-frontend function for meta keywords.
• Added an upgrade function for 6.3 that removes the related settings from the wpseo_titles option.

Test instructions

This PR can be tested by following these steps:

• Meta keywords should be gone everywhere.
• Noindex subpages feature is gone.
• Related options in the wpseo_titles option should be gone, if the upgrade routine has run (for which you have to set your version to 6.3).

Quality assurance

• I have tested this code to the best of my abilities

Fixes: #8753
Fixes: #8709