Make disabling TerraformNotification banner possible

What does this MR do and why?

This MR adds a feature flag show_terraform_banner in order to add the possibility for our customers to turn it off for self-managed.

The feature flag is enabled by default as we want to keep showing the banner.

Screenshots or screen recordings

No visual changes. Screenshot for the context:

How to set up and validate locally

To validate locally:

1. Create a project that contains the Terraform code (the .tf files should be in the top 5 file types)
2. Go to the project's main screen and see the banner
3. Disable the show_terraform_banner feature flag and verify the banner is not shown anymore

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 #341836 (closed)