edit GitLab license info so that GitHub recognizes it

What does this MR do?

This commit updates the LICENSE file so that it contains the exact text of the MIT license. It also moves the reference to third-party software licensing to the README.

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

No.

Why was this MR needed?

GitHub uses a library called Licensee to identify a project's license type. It shows this information in the status bar and via the API if it can unambiguously identify the license.

These changes allow Licensee to successfully identify the license type of GitLab's codebase as MIT.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

I think that this MR meets the relevant two acceptance criteria. But this is my first GitLab-CE MR ever, so please correct me if I'm wrong!

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

N/A