Fix https://github.com/go-gitea/gitea/pull/29874#discussion_r1542227686
- The migration of v292 will miss many projects. These projects will
have no default board. This PR introduced a new migration number and
removed v292 migration.
- This PR also added the missed transactions on project-related
operations.
- Only `SetDefaultBoard` will remove duplicated defaults but not in
`GetDefaultBoard`
(cherry picked from commit 40cdc84b368cce8328b4b49ea5ecf1c5fa040300)
