Add sniff to detect @deprecated tags that are older than 2 major versions.

[omarreiss]

I'd like to see a WordPress plugin sniff that reads the plugin version and detects functions and classes that have been @deprecated more than 2 major versions ago. Let's also throw an error when @deprecated tags don't contain a parsable version.

[jrfnl]

So, there are basically three issues which we see if we read this ticket:

1. Flag the use of deprecated WP Core functions and such. That is already handled by WPCS.
2. Flag functions deprecated in the Yoast plugins (more than x version ago) to be removed. This is the only bit which could (should) be addressed in YoastCS, but would need clear specifications on how to determine what to flag.
3. Formatting/description check of the @deprecated tags. This should be addressed in a separate Docs standard.