Rename `Backlog` column to something else

Description

• The Backlog column in an issue board is confusing.
• The word Backlog is a term specific to Agile workflows. And it's not clear that that column is still scoped to the board config when we use that term. That term is not used elsewhere in GitLab.
• So we should use a term that is less confusing.
• I suggest we use Open or Open issues to simply indicate that the column contains open issues. And I don't think there would be confusion that if you move issues from the Open column to other columns that you would be closing them necessarily.

Proposal

Change the Backlog list name to Open. This will provide symmetry with the Closed list.