Ee addition for : 48834 chart versions for applications installed by one click install buttons should be version locked

What does this MR do?

Update ee/app/models/ee/clusters/applications/prometheus.rb to use VERSION

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Discovered another place in EE where we use version when we should use VERSION

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • [-] Has been reviewed by a UX Designer
  • [-] Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • [-] Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [-] Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

Manual QA to do

• Create a cluster on master, install prometheus
• restart gdk, then upgrade prometheus with this MR's code

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/48834