Limit count of merge requests on the list view

What does this MR do and why?

Limits the count in the tabs on the merge request list to improve performance. Currently we will load the full count of merge requests for the project or group which can get expensive with more merge requests.

This change limits the number to show 999+ if there are more than 999 merge requests in the project or group.

References

#563367 (closed)

Screenshots or screen recordings

How to set up and validate locally

To test you can either visit a project that has more than 999 merge requests

If you do not have this many locally you can change https://gitlab.com/gitlab-org/gitlab/-/blob/df84e9cc694a27fd6df49ca446d6d9e510662325/app/assets/javascripts/merge_requests/list/components/merge_requests_list_app.vue#L112 to a lower number.

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