Discourage usage of the --rps option in favor of arrival-rate

[na--]

--rps was an old k6 option that existed before the arrival-rate executors. It is very limited, it doesn't work well in the cloud (each instance has the same rps value, it isn't segmented) and has a ton of other surprising effects. Some examples of the confusion:

• Iteration duration is calculated incorrectly when vus's are more than requests per second k6#1631
• Support for gRPC protocol  k6#1623 (comment)
• Segment --rps option between instances k6#1341

It's probably still a bit early to deprecate the option, but we should at least link to arrival-rate executors everywhere we mention the rps.