Draft: Add custom sign in text to LDAP sign in form

What does this MR do and why?

(Based on #376698:). After upgrading to GitLab 15.4.1 (CE) the custom sign-in text (GitLab Admin > Settings > General > Sign-in restrictions) is only shown on the Ssandard login form, but not on the LDAP login form.

The documentation of the custom sign-in text states "Your users see the Custom sign-in text when they navigate to the sign-in screen for your GitLab instance."

I assume that the Custom Sign-in text should be displayed for standard sign-in and for LDAP sign-in.

Closes #376698.

Related to #378004 (closed).

Todo list

• Add custom sign-in text to the LDAP sign-in form
• Add rspecs covering the changes

Screen recordings

Before: Custom Sign-In text is not shown in LDAP Sign-In form	With this MR applied: Custom Sign-In text is shown in LDAP Sign-In form
Custom_sign-in_text_missing_in_tab_LDAP	Custom_sign-in_text_in_tab_LDAP

How to set up and validate locally

1. Enable LDAP sign-in in config/gitlab.yml (See config/gitlab.yml.example line 710)
2. Start GitLab (gdk start)
3. Enter a custom sign-in text
   1. Open GitLab (http://127.0.0.1:3000/) and login as user with admin permissions (like root)
   2. Go to Admin > Settings > General > Sign-in restrictions > Sign-in text and enter a custom sign-in text
   3. Sign-out of your local GitLab (http://127.0.0.1:3000/users/sign_out)
4. Open user sign-in form (http://127.0.0.1:3000/users/sign_in)
   1. Click tab "Standard". Below the "Sign in" button, you should see your custom sign-in text.
   2. Click tab "LDAP".
      Without this MR, you should not see your custom sign-in text in tab "LDAP".
      With this MR, you should see your custom sign-in text in tab "LDAP".

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #376698