Room versions 8 and 9: Restricted rooms (#3387)

* Room versions 8 and 9: Restricted rooms

MSCs:
* https://github.com/matrix-org/matrix-doc/pull/3083
* https://github.com/matrix-org/matrix-doc/pull/3289
* https://github.com/matrix-org/matrix-doc/pull/3375

* Changelogs

* Capitalization

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Remove verbiage for spaces because they don't exist

* Iterations on text

* Another clarification

* Make error code descriptions consistent

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Incorporate from merge

* Misc language update per review

* Update accuracy before splitting auth rules

* fix wtf moment

* Fix up v8 and v9 to match "fully specify room versions"

* Scope auth events selection to room version

* Apply consistency

* Add changelogs

* Review part 1

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Split out redaction sections

* Clarify general case of join conditions

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

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