Remove Gemnasium badge from project's README.md

What does this MR do?

Remove Gemnasium badge from project's README.md.

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

No.

Why was this MR needed?

For this project, we (Team, Core team and non-Core contributors) do not need the follow-up docs, while Gemnasium was completely shut down and the following docs were prepared.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Conform by the code review guidelines
  • [n/a] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [n/a] Has been reviewed by a Backend maintainer
  • [n/a] Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• [n/a] If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #45404 (closed)