Move PRs with "backport-done/X.Y" into the "done" column for the current version

[joestringer]

release/cmd/projects/projects.go

Line 318 in a74d624

When performing the release, occasionally there are PRs in the "needs-backport" column that have the "backport-done/X.Y" label. If the backport is done, these cards should be moved to the "backport done" column in the existing project before closing it.

[joestringer]

I think the background to why this happens is roughly this order of events:

• Someone creates a backport PR (which gets added to a project)
• The backport does not land in time for the release
• A new release is cut, new projects are created, and the branch is updated to point to the new projects in the MLH bot configuration
• The backport PR gets merged. The labels are updated. However MLH bot doesn't understand that the PR should be moved to the "done" column for the new project because the base branch for the backport PR was from before the release, ie it points to the old closed project.

(Not quite exactly the above, because if this is the case then the PRs would be in the "backport-pending" column of the old project, but the case I observed recently was that PRs with labels "backport-done" were in a new project in the "needs-backport" column.)