cc53c96bf8
...to try to control memory usage. `HomeServerConfig`s hold on to many Jinja2 objects, which come out to over 0.5 MiB per config. Over the course of a full test run, the cache grows to ~360 entries. Limit it to 8 entries. Part of #15622. Signed-off-by: Sean Quah <seanq@matrix.org>