Remove "Upgrade 🚀" item in top-right profile dropdown

What does this MR do and why?

Removes the “Upgrade 🚀” link from the user dropdown menu as per the recommendation outlined in Remove "Upgrade 🚀" item in top-right profile dr... (#352057 - closed).

Screenshots or screen recordings

Before	After
Note: The “Start an Ultimate Trial” link is also being shown here due to it being cached for 10 minutes

How to set up and validate locally

1. Have a working CustomersDot application which is also connected to Zuora & integrated with your local GitLab install
2. Use GitLab in SaaS mode
3. Ensure that billing plans are enabled
4. Create a new user, group, & project
5. Navigate to the newly created group
6. Select Settings › Billing from the sidebar
7. Click Upgrade on the Premium plan
8. Fill out the in-app checkout form using dummy credit card data
   • One gotchya I ran into was if I had a mismatch between my selected state (in the US) and the zip code I entered
9. Once you land back on the group page, there should be no “Upgrade 🚀” menu item in your user’s dropdown menu in the top navbar

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352057 (closed)