[MSSQL] Metricbeat documentation needs update

[muthu-mps]

As the mssql metricbeat module is already moved to ga. This sentence below is not relevant. The documentation has to be updated.

This is the Microsoft SQL 2017 Metricbeat module. It is still under active development to add new Metricsets and introduce enhancements.

[bizmate]

why dont you add a PR to fix it? Documentation is rather simple

[botelastic[bot]]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!