doc: releases: unify disjoint issue related sections for v3.1.0

There are three lists of issues in the release notes:

- security vulnerabilities which received CVEs
- other open bugs
- resolved issues

(Here, "issue" is a strictly more general term than "bug": every bug
is an issue, not every issue is a bug. This is GitHub-specific
jargon.)

The current structure is awkward. Clean it up by unifying the
lists under a single top level section and cleaning up the distinction
between issues and bugs.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>