...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>