Feat/4211: Related Posts: Add setting to weight results by date

[Sidsector9]

Description of the Change

• Fixes a bug where Related Posts were sorted by date instead of by Elasticsearch relevance score, causing more_like_this scoring to be ignored. See comment by @mckdemps in Related Posts not working as expected #1841, also related to Related posts show latest published posts #1865, "Related Posts" sometimes returns latest posts #1274.
• Adds a new "Weight related posts by date" checkbox setting to the Related Posts feature, applying a date decay function to favor newer content. (Disabled by default, but I strongly believe it should be enabled. Open to discussion)

Default decay values

The defaults are intentionally more conservative than Search's weight_recent() because related posts have a longer useful lifespan, a well-written article from several months ago is still a good recommendation, unlike search results where recency matters more.

• Scale (360d vs Search's 14d): score decays over a full year instead of two weeks
• Offset (30d vs Search's 7d): posts within a month are completely unpenalized
• Decay (0.5 vs Search's 0.25): at the scale distance, score is halved rather than quartered (gentler)

Closes #4211

How to test the Change

1. Add a lot of dummy related posts to your site, where the created date spans over at least 3 years.
2. Go to ElasticPress > Features > Core Search > Related Posts.
3. Verify the new "Weight related posts by date" checkbox setting is visible.
4. With the setting disabled, view a post with the Related Posts block or widget. Note the order of results. They should be based on content relevance only, with old posts appearing freely.
5. Enable the "Weight related posts by date" setting and save.
6. View the same post again. Newer related posts should now be favored over older ones in the results.
7. Disable the setting again and confirm the results return to pure relevance ordering.

Changelog Entry

Added: "Weight related posts by date" setting to the Related Posts feature, allowing newer content to be favored in results using Elasticsearch's date decay function.
Fixed - Related posts being sorted by date instead of relevance score, causing more_like_this scores to be ignored.

Credits

Props @burhandodhy @Sidsector9

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.