Event reports have no character limit

This issue has been migrated from [#11663](https://github.com/matrix-org/synapse/issues/11663).

---

### Description

Hi, I had a user send a report that is over 10,000 characters long and it occurred to me there is no character limit on reports. I think this could lead to all sorts of problems from inconvenience, to filling up my disk with useless info.

There should be some kind of character limit to reports. I think 500 characters should be plenty for any kind of report.
### Version information

- **Version**:
"server_version":"1.49.2","python_version":"3.9.2"
- **Install method**:

Official Debian package
- **Platform**:

Debian 11 Bullseye