Unpin build-system requirements, but impose an upper-bound (#14085)

* Revert to prior build-system requirements

This reverts #14080.

* Use normalised extra name, which poetry-core 1.3 will generate anyway

* Changelog

* Upper bound build-system requirements

* Remove upgrade note; expand changelog entry a little.

* Fix typo in build-system comment

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

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