docs: add promtool version in rule migration cmd (#6989)
docs: add promtool version in rule migration cmd (#6989)

`promtool update rules` is no longer available after Prometheus 2.5.
Document this to avoid confusion such as in #6988.

Signed-off-by: Christian Hoffmann <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@hoffmann-christian.info>
keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus