Unconfigurable harsh ratelimiting for leaving rooms

This issue has been migrated from [#16006](https://github.com/matrix-org/synapse/issues/16006).

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### Description

Clients (at least Element) have an option to leave a space and leave all rooms in it.


If you choose to leave all rooms, the client will make leave requests for every room you are on.

If you are on many rooms, this will take a long time, to the point it seems like it hangs. Looking at the network tab in the developer tools shows a lot of 429 Too Many Requests responses.

In conclusion, the defualt ratelimiting for leaving rooms is too harsh. It doesn't seem like it can be overriden either:

https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html has mention of `rc_joins` but there is no `rc_leaves`

### Steps to reproduce

- join a space on Element on a homeserver using the default rate limits
- join many rooms
- leave the space, choosing to leave all rooms

### Homeserver

matrix.mit.edu

### Synapse Version

{"server_version":"1.88.0","python_version":"3.10.6"}

### Installation Method

Debian packages from packages.matrix.org

### Database

PostgreSQL

### Workers

Single process

### Platform

Ubuntu 22.04.2, PowerEdge R710

### Configuration

_No response_

### Relevant log output

```shell
N/A
```

### Anything else that would be useful to know?

_No response_