style: adjust login form margins for consistency (#61667)

[im3dabasia]

Trac Ticket

Trac ticket: https://core.trac.wordpress.org/ticket/61667

Brief Description

This pull request addresses Trac Ticket #61667 by adjusting the margins around the login form for visual consistency. The margin above and below the form is now consistently set to 24px, and the margin below the logo has also been adjusted to 24px, as discussed in the Trac.

[github-actions]

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[github-actions]

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[github-actions]

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[audrasjb]

I added a commit to merge margin declarations into a single one.
I think we're good to ship this now :)

[audrasjb]

committed in https://core.trac.wordpress.org/changeset/59551