Read-only render caching on POST requests for dynamic page cache

Problem/Motivation

See 📌 Make POST requests render cacheable Needs work and 📌 Cacheability information from route access checker access results are ignored by dynamic_page_cache Needs review for background.

We added support for read-only render caching on post requests - this means that render elements that are already cached will return from cache during a POST request. We can probably do the same for dynamic page cache too, since it will handle form placeholdering automatically so should work the same way. If not, only one way to find out.

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Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet