Add username to terms message in git and API calls

What does this MR do?

Add username to terms message in git and API calls

This will make it clearer to users which account is being used to make the API/git call. So they know which account needs to be used to accept the terms.

Hopefully this will cut down on support requests a bit.

The message now reads:

You (@reprazent) must accept the Terms of Service in order to perform this action.
Please access GitLab from a web browser to accept these terms.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes #46649 (closed)