[login] Consider removing separate form part and using role on the wrapper

[web-padawan]

Motivation

Currently, vaadin-login-form uses the following DOM structure internally:

web-components/packages/login/src/vaadin-login-form-wrapper.js

Lines 61 to 62 in ebf5367

	<section part="form">
	<div part="form-title" role="heading" aria-level="${this.headingLevel}">${this.i18n.form.title}</div>

This could be simplified in V25 by making vaadin-login-form-wrapper use role="region" instead.

Proposed solution

• Remove <section> and set role="region" on the vaadin-login-form-wrapper element
• Replace part="form-title" with a slotted title and set aria-labelledby on the form wrapper