[__jitsi_meet_domain] Add a muc_room_cache_size for jibri

@pedro is working on this and this change matched my workflow better :-)

type/__jitsi_meet_domain/files/prosody.cfg.lua.sh

@@ -153,6 +153,8 @@ Component "internal.auth.${JITSI_DOMAIN:?}" "muc"
     admins = { "${FOCUS_USER:?}@auth.${JITSI_HOST:?}", "jvb@auth.${JITSI_HOST:?}" }
     muc_room_locking = false
     muc_room_default_public_jids = true
+    -- https://prosody.im/doc/modules/mod_muc
+    muc_room_cache_size = 1000
 ${PROSODY_DOMAIN_END}
 ${PROSODY_MAIN_START}
 -- This will be managed by __jitsi_meet