Merge branch 'issue-boards-issues-total-count' into 'master'

Add the total number of issues in the JSON response in issue board lists

## What does this MR do?

Add the total number of issues in the JSON response in issue board lists

## Why was this MR needed?

The issue board lists should always show the total number of issues in the list, not the current amount fetched by endless scroll.

Closes #21327

See merge request !5904