Add 8px margin below the `Remember me` checkbox

What does this MR do and why?

In the sign in page, increase the margin between the Remember me checkbox to the Sign in button by adding the wrapper .gl-mb-3, so that the total distance between the checkbox and the button 16px.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

This spacing aligns better with the rest of the sign in forms:

Screen_Recording_2024-01-29_at_12.52.46

How to set up and validate locally

1. On an incognito window, go to /users/sign_in
2. Check bottom margin of Remember me checkbox.