Make the 'Accepting merge requests' workflow consistent

According to gitlab-org/quality/triage-ops#70 (comment 109494018), we are changing the following:

1. We keep using the Accepting merge requests label to track issues open for contributions
2. The Backlog (Accepting Merge Requests) milestone is renamed to just Backlog
3. Make the case of the Accepting merge requests label consistent across all the occurrences

This goes hand-in-hand with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22467.

Note: the Accepting merge requests label will automatically be added to all the scheduled issues in the future, see gitlab-org/quality/triage-ops#70 (closed).

/cc @JobV @bikebilly @jeremy_ @meks @sarrahvesselov @markglenfletcher