Replace cache_in_self with functools.cached_property

[ShadowJonathan]

This acts a loss less magic-y and more pythonic by storing the result of its value as a direct name mapping in the instance itself.

In other words, this'll make the property function run once, and then store its value as the same name of the property, which can then be read and written to like any normal value.

To me, this looks a loss less hack-y than cache_in_self, which currently does it with underscore-prefixed values.

However, this is only available to synapse once 3.8 is the lowest supported python, see the docs

[callahad]

While we wait for Python 3.7 to age out (Summer 2023), I'd be game to see what a pull request using backports.cached-property looks like.

Rich notes that our cache_in_self does work to detect cycles which may not be happening in the stdlib functools.cached_property