pyrsistent "transitive dep" is problematic

### Background

*As discussed in the [Synapse Package Maintainers](https://matrix.to/#/!rh9Uxk45AsPongyP3ypgpsCmuIufiggD6mDXFWh4_FM/$bhDimbyDN2TIGtzN04s_6Kr9ulGwq5og1Rgt2rEVXdI?via=jki.re&via=matrix.org&via=element.io) room,*

`pyproject.toml` declares synapse's dependencies. In addition to what synapse actually depends on there are "transitive dependencies". These are a least-bad approach for avoiding user problems (and bug reports!) because A that Synapse depends on is a version which is good enough, except that it depends on B, and the B installed was good enough for what A said at release, except that it actually isn't good enough for correct behavior. Synapse therefore adds a dependency on new-enough B. This is all merely background.

### Description

One of the "transitive dependencies" is `pyrsistent`, said to be for `jsonschema`. However, `jsonschema` dropped `pyrsistent` as a dependency in `4.18.0`, summer of 2023. People who are up to date then won't need `pyrsistent`. But Synapse only requires `jsonschema` > 3, and people with ancient `jsonschema` might have problems.

My suggestion is to require `jsonschema` >= 4.18.0, and drop the `pyrsistent` transitive dependency. (I realize that wades into the supporting LTS question, but I don't understand wanting new synapse and old everything else.)

### Steps to reproduce

Read the source code and then go read lots of other repo history.

### Homeserver

reading.the.code

### Synapse Version

1.148.0

### Installation Method

Other (please mention below)

### Database

none

### Workers

I don't know

### Platform

Discovered during packaging by code reading.

### Configuration

_No response_

### Relevant log output

```shell
N/A
```

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

_No response_

Start with the dependency declarations in pyproject.toml, then review the repository history and supporting-LTS discussion mentioned in the issue. Confirm the supported jsonschema versions and whether jsonschema >=4.18.0 is acceptable; done means removing the pyrsistent transitive dependency while preserving the project's compatibility policy.

Written by the indexing model from the issue text.