Datasource: Show access (Browser/Server) select on the Prometheus datasource

[jorgelbg]

What this PR does / why we need it:

Since the datasource configuration of the Prometheus datasource was migrated to React on PR #20248 the access options (Browser/Server) are no longer visible/configurable.

I found that in #12757 there was a proposal of removing entirely the Browser/Direct option (for Elasticsearch) but I could see the reasoning extending to all other datasources as well. Nevertheless, I didn't find anything specifically about the removal of the options in the CHANGELOG and the documentation for the current release (v6.6.0) still mentions the Access option.

Also, the ES datasource was already migrated to React and kept the Access option as well.

Special notes for your reviewer:

If the final decision of the Grafana team is to remove the option altogether feel free to close this PR. In that case, a new PR updating the documentation should be opened.

[peterholmberg]

I think this is a mistake from me, good catch! Could you fix the failing test so we can merge this?

[jorgelbg]

@peterholmberg I retriggered the CI pipeline because the test that was failing was not from the modified code (the ES datasource was the one failing). All the tests passed now 🤷‍♂.

P.S: I suspect that it could have been related to the time management on the ES datasource (with the index pattern) but couldn't reproduce it either.

[marefr]

@peterholmberg this is 6.6.1 material so have updated milestone and labels