WordPress plugins: update the polylang cookie solution

[jkudish]

Summary

WordPress Plugins and Themes with Known Issues: Polylang - This PR seeks to update the proposed solution to the Polylang's cookie preventing cache hits. The previous solution removed the polylang cache key. The new solution makes the cookie a cache-varying cookie which is a better and more reliable solution.

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Post Launch

Do not remove - To be completed by the docs team upon merge:

• [ ] Redirect /old-path/ => /new-path/ (if applicable)
• [ ] Include/exclude pages ^ respectively within docs search service provider (if applicable)
• [ ] For Heroes - add a props post to the discussion board.
• [ ] Remove from the project board

[rachelwhitton]

@ryanshoover is there someone on your team you can assign to provide a technical peer review here?

[stevector]

Thanks @theambert! I'll merge

[stevector]

Assigning to @rachelwhitton to look at the screenshot

[pantheon-decoupled[bot]]

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