Update Warmer API documentation to indicate it is deprecated

[inqueue]

The Warmer API has been deprecated in 2.x and will be removed in 5.0 (#15607). Please update product documentation to indicate status.

[jasontedor]

This is covered in the migration docs and redirects which say:

Warmers have been removed. There have been significant improvements to the index that make warmers not necessary anymore.

The pull request #15614 shows additional changes to the docs. Do you think that this isn't enough?

[inqueue]

There are a lot of good changes to the future docs. Do you think we need something in 2.x documentation to indicate it is deprecated and going away entirely?

[jasontedor]

Do you think we need something in 2.x documentation to indicate it is deprecated and going away entirely?

Yeah, I don't see anything there and you're right that something should be added. Thanks for raising this.

[jpountz]

OK I will add a deprecation warning.

[jpountz]

Done.