Document and ensure upgrade path to 7.0

What does this MR do?

Create a release info page for 7.0 and document the upgrade path to 7.0. Validate upgrades follow this upgrade path.

Related issues

Closes #4522 (closed)

🔍 Test Plan

Not Supported Upgrade Path

1. Install GitLab chart < 15.11
2. Upgrade to chart 7.0/GitLab 16.0 (emulate by setting appVersion/version in Chart.yaml)
3. Verify the message

   It seems you are upgrading the GitLab Helm Chart from 6.10.0 (GitLab 15.10.0) to 7.0.0 (GitLab 16.0.0).                                     
   It is required to upgrade to the latest 6.11.x version first before proceeding.                                                             
   Please follow the upgrade documentation at https://docs.gitlab.com/charts/releases/7_0.html                                               
   and upgrade to GitLab Helm Chart version 6.11.x before upgrading to 7.0.0.

   is printed by the upgrade-check Job. The Job should fail and no workloads should be updated to newer images.

Supported Upgrade Path

1. Install GitLab chart 15.11 (emulate by setting appVersion/version in Chart.yaml)
2. Upgrade to chart 7.0/GitLab 16.0 (emulate by setting appVersion/version in Chart.yaml)
3. Verify the upgrade-check Job succeeds.

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
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
  • gitlab-org/omnibus-gitlab#7745 (closed)
• 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.