Fix a long-standing spec compliance bug where Synapse would accept a trailing slash on the end of `/get_missing_events` federation requests. (#13789)

* Don't accept a trailing slash on the end of /get_missing_events

* Newsfile

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

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