Limited message history unless I increase federated room backfill limit

### Description:

I tried creating a new server and setting up federation between the old and the new servers. When a user is invited to a federated room, they don’t see the full room history — it seems that federation only provides a limited amount of history (about 100 messages). The room’s history visibility is set to “since the point in time this option was selected”, so I’m not referring to rooms where history visibility is set to “since they were invited” or “since they joined.”

After checking the Synapse source code, specifically `synapse/handlers/federation.py`, I found that the function `try_backfill` has a hardcoded `limit=100`.

https://github.com/element-hq/synapse/blob/dc7f01f334aa8aa92ca10cdf2e8de64a830effa4/synapse/handlers/federation.py#L446-L540

### Potential solutions

Would it be possible to change this limit through `homeserver.yaml`? Or is there a specific reason why this value is hardcoded?