Reword Securing GitLab flip negative to positive

What does this MR do?

Updates the text regarding security recommendations to turn negative into positive.

Current text may be interpreted as suggesting defaults are insecure and malicious user creation is a known problem.

Updated text focuses on features available for access control needs and reducing exposure/risk.

Screenshots

Does this MR meet the acceptance criteria?

Yes!

Context

This MR (and it's sister MR) are the product of discussion in AppSec Office Hours and a chat with @ankelly about making an MVC that everyone can (hopefully) agree on.

Requested feedback and discussed this MR in AppSec Office Hours. There is ongoing discussion on other ways to increase awareness of this and/or use secure defaults, but seemed to be a consensus that this MVC is a step in the right direction.

@axil @mikelewis @jcolyer Ready for review! Any feedback/suggestions appreciated.