`provisioning.prefix` config is not applied as of v0.25.0

When using the provisioning API, the listening URL endpoint prefix is hardcoded to `/_matrix/provision` instead of whatever prefix was set in the config via `provisioning.prefix`. This happened in v0.25.0 with the replacement of the HTTP routers ([`d5223cd`](https://github.com/mautrix/go/commit/d5223cdc8fcebace5f1de8d9b02f4f4568fc663d#diff-bb2303cf2c75c8cdc6eb98d731864035a5bb4ac462abea969233bd0a3a3a422cR137-R160)). Either the config value should take effect, or it should be removed from example files & the config struct to make it clear that it's no longer supported.