Config should yell and warn about float values being cast to ints

Wherever an `int` is used for a config value, if someone accidentally provides a `float` number value, it will get floored (rounded down).

This can lead to misconfigurations like specifying `0.5` for the rate limiting [`burst_count`](https://github.com/element-hq/synapse/blob/develop/docs/usage/configuration/config_documentation.md#ratelimiting) which will get rounded down to `0` and not allow requests at all (real example: https://github.com/element-hq/synapse/pull/18159).