Adjust a documentation of the experimental module

[olegbespalov]

What?

In k6 v0.49, the experimental grpc module becomes part of the k6-core. We decided to keep the possibility of importing the module till k6 v0.51, but it's good to warn customers in old documentation that the experimental module will be removed entirely.

So once k6 v0.49 is released, we have to update the old version of the documentation and offer the customer to upgrade to the latest k6 and use k6/net/grpc instead.

Why?

We help our customers to prepare for migration.

Related: #1472