[EE] Make the 'Accepting merge requests' workflow consistent

EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22467.

What does this MR do?

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 gitlab-com/www-gitlab-com!15622 (merged).

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).

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder