Fix incorrect issueable counts

What does this MR do?

This MR fixes the incorrect issuable counts which was caused by the optimized_issuable_label_filter feature flag (improves the label filters on issuables).

2 bugs:

• On the Dashboard page, #count_by_state method falls back to the original implementation in IssuableFinder, however the base query is still coming from the OptimizedIssuableLabelFilter module
  • Fixed by removing the namespace check (this was originally added when the optimization was only working for group level queries). Safe to remove.
• On any issuable page, filtering by approved_by_usernames will do a GROUP BY query and returning several rows. Merging the returned rows was not implemented in OptimizedIssuableLabelFilter#count_by_state. The module was introduced before this filter. The implementation is now the same as in IssuableFinder.

Added 2 extra test cases which were failing before applying the fix.

Note: OptimizedIssuableLabelFilter will be merged into IssuableFinder in 14.0 🤞

Does this MR meet the acceptance criteria?

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