Remove Helm v2 usage from documentation

What does this MR do?

Removes references to Helm v2 in charts documentation. Only Helm v3 is supported for GitLab 14.0.

Related issues

Closes #2726 (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

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened

Initial list from issue:

• advanced/external-mattermost/index.md
• advanced/geo/index.md
• advanced/persistent-volumes/index.md
• charts/gitlab/migrations/index.md
• development/deploy.md
• development/minikube/index.md
• index.md
• installation/deployment.md
• installation/index.md
• installation/migration/helm.md
• installation/tools.md
• installation/version_mappings.md
• releases/3_0.md - historical, not edited
• releases/4_0.md - historical, not edited
• troubleshooting/index.md