Make 15.0 minimum required version to upgrade to 15.x

Make 15.0 minimum required version to upgrade to 15.x

Prevents upgrade if the currently installed package is not
at least 15.0.x.

Changelog: changed

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

What does this MR do?

Make 15.0 minimum required version to upgrade to 15.x, following https://docs.gitlab.com/ee/update/#upgrade-paths

Previous changes

• Enforcing 14.10 to upgrade to 15.0 - !6050 (merged)
• Enforcing 14.0 to upgrade to 14.x - !5398 (merged)

Related issues

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

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 the GitLab Chart opened