User #placeholder_strategy_denylist for CacheOptionalInterface blocks to prevent them being rendered by BigPipe

Problem/Motivation

📌 Make language switcher block cacheable Postponed prevents the language switcher block being cached in the dynamic page cache.

We can also prevent it being rendered by bigpipe using #placeholder_strategy_denylist, this will mean that after 📌 Allow big pipe to run for session-less users Active it won't prevent pages going into the page cache (external or internal).

Additionally because the language switcher block is often placed at the top of the page and is very cheap to render, it will additionally prevent jank/content layout shift issues from it being rendered by JavaScript.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet