Remove redundant h4 from pagination.twig for improved accessibility

Problem/Motivation

In #3232222: Pager template h4 can be out of document hierarchy order it's proposed to remove a <h4> element from the navigation, since the h4 is redundant and can trigger a11y warnings by being out of document order.

This same improvement applies to Radix, and IMO there's no reason to not land it here ahead of in core.

See https://git.drupalcode.org/project/radix/-/blob/5.0.x/src/components/pag...

Steps to reproduce

- See #3232222: Pager template h4 can be out of document hierarchy order, or
- Use Devel Generate to generate 50 nodes
- View the default /node view
- Observe that the headings in the document run h1 (page title), h2 (node titles), h4 (pagination), OR
- Run aXe Devtools or other a11y checker

Proposed resolution

Briefly, replace <nav class="{{ utility_classes|join(' ') }}" role="navigation" aria-labelledby="pagination-heading"><h4 id="pagination-heading" class="visually-hidden">{{ 'Pagination'|t }}</h4> with <nav class="{{ utility_classes|join(' ') }}" role="navigation" aria-label="{{ 'Pagination'|t }}">

Remaining tasks

- Patch, review, discuss, merge etc

User interface changes

Improved accessibility.

API changes

Data model changes