Reduced error level to default for not found post error

refs #12064

- `critical` is meant to be something unpredictable like internal error, something worthy attention, as described in Ignition -3439456d94/README.md (list-of-errors)
- This error level was introduced with - this PR https://github.com/TryGhost/Ghost/pull/9426, but there is no context provided why this specific value was used. Assuming it's an outdated value as 'not found' is nowhere to be treated in any special way

core/server/models/relations/authors.js

@@ -353,7 +353,6 @@ module.exports.extendModel = function extendModel(Post, Posts, ghostBookshelf) {
                     .then(function then(foundPostModel) {
                         if (!foundPostModel) {
                             throw new errors.NotFoundError({
-                                level: 'critical',
                                 message: i18n.t('errors.models.post.postNotFound')
                             });
                         }