Remove "Withdrawn" status for OEPs

[sarina]

The only OEP that uses the Withdrawn status is https://docs.openedx.org/projects/openedx-proposals/en/latest/best-practices/oep-0013.html and I propose we remove this status.

It is sane to say that the Closed PRs list (specifically, is:pr is:closed is:unmerged is a good representation of what's withdrawn.

Criteria for this ticket include:

• Edit OEP-1 to remove the Withdrawn section
• Edit OEP-1 to add a section under OEP Maintenance that explains that "some OEPs have been written but ultimately abandoned before merge, they will be found in closed unmerged prs, here's a list of them"
• Add Changelog entry to OEP-1
• Change status of OEP-13 to Obsolete