The manhole's hardcoded private key uses a very outdated signature type

This issue has been migrated from [#12629](https://github.com/matrix-org/synapse/issues/12629).

---

The manhole has a hard-coded private key (https://github.com/matrix-org/synapse/issues/3850) which uses the `ssh-rsa` signature type. This refers to a combination of RSA and SHA-1, and is now quite outdated.

In fact it's so outdated, that newer versions of OpenSSL will now refuse to connect:

```
$ ssh -p9000 matrix@localhost
Unable to negotiate with ::1 port 9000: no matching host key type found. Their offer: ssh-rsa
```

(`ssh-rsa` was [deprecated in OpenSSL v8.2](https://www.openssh.com/txt/release-8.2), and [disabled in OpenSSL v8.8](https://www.openssh.com/txt/release-8.8)). We should (at least) use a key with a more up-to-date signature algorithm.

---

A workaround, if needed, is to add the following to your `~/.ssh/config`:

```
Host
PubkeyAcceptedAlgorithms +ssh-rsa
HostkeyAlgorithms +ssh-rsa
```