Stop promoting k6/experimental/grpc

[olegbespalov]

What?

In k6 v0.49, we graduated the experimental gRPC module, and we're going to remove it. So this change warns customers (it's already part of the v0.49 see https://grafana.com/docs/k6/v0.49.x/javascript-api/k6-experimental/grpc/).

Checklist

Please fill in this template:

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• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the make docs command locally and verified that the changes look good.

Select one of these and delete the others:

If updating the documentation for the most recent release of k6:

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant (e.g. when correcting a documentation error) folders of the previous k6 versions of the documentation.

If updating the documentation for the next release of k6:

• I have made my changes in the docs/sources/next folder of the documentation.

Related PR(s)/Issue(s)

#1472

Closes #1473