Update robots.txt to exclude group_members and project_members, which can expose…

What does this MR do?

Modifies robots.txt so that group_members and project_members pages are not caught up by web-crawlers. This effectively blocks exposure of user-ids and names to web-crawlers, which might be sensitive information.

What are the relevant issue numbers?

No issue created.

Does this MR meet the acceptance criteria?

• [ NN ] Changelog entry added, if necessary
• [ NN ] Documentation created/updated
• [ NA ] Tests added for this feature/bug
• [ NA ] Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• [ NA ] Conforms to the style guides
• [ NA ] Conforms to the database guides
• [ NA ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer