Resolve "Rename project tags to project topics"

What does this MR do?

1. Rename "Tags" to "Topics" in the project settings.

2. Update documentation, change project tags to project topics.

3. Clean up FE codebase (CSS classes, helpers) by using topics instead of tags where applicable.

Screenshots:

before	after

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54167

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• 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

Closes #54167 (closed)