Only permit 15.0 upgrade from 14.10

What does this MR do?

Only permit 15.0 upgrade from 14.10

- Only allow administrators to upgrade to a chart with GitLab 15.0 if
  they have 14.10 installed
- Update release documentation notes to reflect upgrade path

Closes https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3285

Changelog: changed

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Related issues

Closes #3285 (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