docs: add promtool version in rule migration cmd
`promtool update rules` is no longer available after Prometheus 2.5.
Document this to avoid confusion such as in
@@ -120,7 +120,7 @@ new format. For example:
$ promtool update rules example.rules
```
-Note that you will need to use promtool from 2.0, not 1.8.
+You will need to use `promtool` from [Prometheus 2.5](https://github.com/prometheus/prometheus/releases/tag/v2.5.0) as later versions no longer contain the above subcommand.
## Storage
