Increase default Synapse/rc_message rate limits

The new rate limits are more suitable for running Element Call.

Ref:

- https://github.com/element-hq/element-call/blob/93ae2aed9841e0b066d515c56bd4c122d2b591b2/docs/self-hosting.md#a-matrix-homeserver
- #3562

Instead of trying to increase them conditionally only when Element Call
is enabled, it's simpler and good-enough to just increase them in all cases.

Author: spantaleev

roles/custom/matrix-synapse/defaults/main.yml

@@ -416,8 +416,8 @@ matrix_synapse_root_log_level: "WARNING"
 
 # Rate limits
 matrix_synapse_rc_message:
-  per_second: 0.2
-  burst_count: 10
+  per_second: 0.5
+  burst_count: 30
 
 matrix_synapse_rc_registration:
   per_second: 0.17