Fix a bug introduced in 1.54.0rc1 which meant that Synapse would refuse to...

Fix a bug introduced in 1.54.0rc1 which meant that Synapse would refuse to start if pre-release versions of dependencies were installed. (#12177)

* Add failing test to characterise the regression #12176

* Permit pre-release versions of specified packages

* Newsfile (bugfix)

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>