Resolve "Specify correct minimum k8s version in documentation"

What does this MR do?

Update the docs to specify:

• the minimum k8s version (1.19) in the docs when deploying the chart with its default values.
• the required kubectl version (1.18 for 1.19 clusters, see: kubectl version skew policy).
• disabling the nginx ingress controllers subchart allows the chart to run on 1.16+.

Related issues

Closes #3052 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
  • not applicable
• Documentation created/updated
• Tests added
  • not applicable
• Integration tests added to GitLab QA
  • not applicable
• Equivalent MR/issue for omnibus-gitlab opened
  • not applicable
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.
  • not applicable