Latest changes from importing/exporting from Grafana 12.3.1 (#19381)

These are automatic changes from importing/exporting from Grafana
12.3.1.

In order to verify that I'm not sneaking in any changes, you can follow
these steps to get the same output.

Reproduction instructions:

 1. Start [Grafana](https://hub.docker.com/r/grafana/grafana)
    ```
docker run -d --name=grafana --add-host
host.docker.internal:host-gateway -p 3000:3000 grafana/grafana
    ```
1. Visit the Grafana dashboard, http://localhost:3000/ (Credentials:
`admin`/`admin`)
 1. Import the Synapse dashboard: `contrib/grafana/synapse.json`
1. Export the Synapse dashboard. On the dashboard page -> **Export** ->
**Export as code** -> Using the **Classic** model -> Check **Export for
sharing externally** -> Copy
 1. Paste into `contrib/grafana/synapse.json`
 1. `git status`/`git diff` to check if there is any diff

Sanity checked the dashboard itself by importing the dashboard on
https://grafana.matrix.org/ (Grafana 10.4.1 according to
https://grafana.matrix.org/api/health). The process-level metrics won't
work because https://github.com/element-hq/synapse/pull/19337 just
merged and isn't on `matrix.org` yet. Also just generally, this
dashboard works for me locally with the
[load-tests](https://github.com/element-hq/synapse-rust-apps/pull/397)
I've been doing.


### Motivation

There are few fixes I want to make to the Grafana dashboard and it sucks
having to manually translate everything back over because we have
different formatting.

Hopefully after this bulk change, future exports will have exactly what
we want to change.

changelog.d/19381.misc

@@ -0,0 +1 @@
+Update Grafana dashboard syntax to use the latest from importing/exporting with Grafana 12.3.1.