Change default room version to 10. Implements MSC3904 (#14111)

* Change Documentation to have v10 as default room version

* Change Default Room version to 10

* Add changelog entry for default room version swap

* Add changelog entry for v10 default room version in docs

* Clarify doc changelog entry

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

* Improve Documentation changes.

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

* Update Changelog entry to have correct format

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

* Update Spec Version to 1.5

* Only need 1 changelog.

* Fix test.

* Update "Changed in" line

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Patrick Cloke <patrickc@matrix.org>