Remove ChunkedToXContentBuilder

[original-brownbear]

Reverts the introduction of the ChunkedToXContentBuilder to fix the various performance regressions it introduced and the theoretical impossibility of fixing its performance to rival that of the iterator based solution.
With the exception of a few minor adjustments that came out of changes already made on top of the builder migration this simply returns to the previous implementations (and some of the stuff in that code could be done better with the utilities available now).
I also verified that this solves the performance issues that we've been running into with the builder.

closes #118647

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Pinging @elastic/es-distributed-coordination (Team:Distributed Coordination)

[elasticsearchmachine]

Hi @original-brownbear, I've created a changelog YAML for you.

[DaveCTurner]

LGTM (assuming CI is happy anyway)

I haven't really contemplated alternatives, but I honestly don't think the API based around Iterator is meaningfully harder to use (or read) than the builder-based one. It seemed a little neater at the outset but clearly has performance issues that aren't easy to solve, and we can't reasonably keep such a significant regression in place simply for fairly subjective code-aesthetics reasons even while we work towards an eventual solution that is both performant and pleasing.

[original-brownbear]

Thanks David!

[thecoop]

@original-brownbear This has not been backported to 8.x...