FIX / Fix authentication settings not being saved

[Mary-Clb]

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes !44007
• This PR fixes the authentication settings form submission.

When saving changes from Configuration > Authentication > Setup, the form posts to config.form.php, which only handles this flow when update_auth is present and non-empty. The submit button was sending update_auth without a value, so the request was not processed by the authentication settings update branch. As a result, the request fell back to the generic configuration page and the changes were not saved.
This change adds an explicit value to the update_auth submit button so the authentication settings save flow is correctly triggered.