Resolve "undefined method `count_of_extra_tags_not_shown'"

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24219 we renamed tags to topics in the FE codebase. One occurrence slipped and wasn't renamed. This MR fixes it.

Rename @project.count_of_extra_tags_not_shown to @project.count_of_extra_topics_not_shown in https://gitlab.com/gitlab-org/gitlab-ce/blob/f5cde3a781a63ebd5630b0eb1fee79fc9750d6be/app/views/projects/_home_panel.html.haml#L26

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/56311

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• 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 #56311 (closed)