Upsell the GitLab Terraform features - iteration 3 backend

What does this MR do?

This MR is a part of changing the GitLab-managed Terraform state banner. As discussed in that MR we want to split the task to ship the change safely for the mixed deployment scenario.

We decided to proceed in 3 steps:

1. Change the design using the old logic
2. Introduce the backend change - current step
3. Then in 14.3 switch to the new implementation using UserCalloutDismisser.

Current MR:

• adds the terraform_notification_dismissed user callout into the app/models/user_callout.rb

Screenshots or Screencasts (strongly suggested)

There are no visual changes at this step. Here is the corresponding banner:

Old design	New design

How to setup and validate locally (strongly suggested)

There will be no call to this callout for now. The corresponding frontend implementation is planned for the next milestone.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #336360 (closed)