Fix LDAP sign in button padding

What does this MR do and why?

A tiny MVC for https://gitlab.com/gitlab-org/gitlab/-/issues/366251 to fix the broken bottom padding in the LDAP sign-in container. We fix the issue by simply adding gl-pb-5 to the submit container.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable LDAP inside your GDK via your config

     # config/gitlab.yml
       development:
         ldap:
         enabled: true
          servers:
            main:
            label: ldap
            host: 127.0.0.1
            port: 3890
            uid: 'uid'
            base: 'dc=example,dc=com'
           ...

2. Navigate to the sign-in page

3. Validate the position of the sign-in button.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #366251 (closed)