Add support for pagination_heading_level in views.

Problem/Motivation

In #3402642 an option was added to set the heading level of full and mini pagers in views configuration. The templates need to be updated as stable currently is hardcoding this to be an h4 here.

Proposed resolution

Update
<h4 id="{{ heading_id }}" class="visually-hidden">{{ 'Pagination'|t }}</h4>
to be something like:
<{{ pagination_heading_level }} id="{{ heading_id }}" class="visually-hidden">{{ 'Pagination'|t }}</{{ pagination_heading_level }}> instead.

I would add a patch for this myself, but my employer’s policies prevent me from contributing to public repositories like GitHub.