How can I change the development version of a module?

Originally, the Link Allowed Hosts module, which I'm a maintainer of (with "administer releases" permission), started with a default branch of 1.0.x. This branch was released and appeared since then as development version 1.0.x-dev.

This was wrong, the default branch needs to be 1.x. So I created a 1.x branch, set it as default on the project's GitLab page and also released it. Still the development version stayed 1.0.x-dev.

I also can't enable this branch in the project's "Releases" page as the matching checkbox is disabled.

How can I fix this? Or where can I file this issue?

There is no way to select that branch as supported, as the Supported checkbox for the 1.* series is disabled by code. That is done by project_release_project_edit_releases() which contains the following code. (drupal.org is still running on Drupal 7.)

// Check if the maintainer is allowed to mark the branch as supported.
if (($supportable_branch_pattern = variable_get('project_release_supportable_branch_pattern_' . $node->type, variable_get('project_release_supportable_branch_pattern'))) && !preg_match($supportable_branch_pattern, $branch)) {
  $form['branches'][$branch]['supported']['#disabled'] = TRUE;
  $form['branches'][$branch]['supported']['#value'] = 0;
  $form['branches'][$branch]['recommended']['#disabled'] = TRUE;
  $form['branches'][$branch]['recommended']['#value'] = 0;
}

That is probably done because the settings for 1.* would also be valid for 1.0.*; using settings for the 1.* and the 1.0.* series would cause "conflicts" between two different settings.

As for in which queue you could file an issue, that is the Project issue queue, the queue for the project that handles projects and their releases on drupal.org. Probably this is seen as works as designed, but the Project maintainers are the only ones who can change that code and decide for any change.

The easiest way to have this resolved is to as in the #drupal-infrastructure channel in Slack.