web-components feat: add root heading level support to dashboard layout

Description

This PR adds root heading level support to DashboardLayout. It was previously added for Dashboard. This PR also refactors the heading level logic in Dashboard, DashboardWidget, and DashboardSection; centralizing them in DashboardLayoutMixin and DashboardItemMixin.

Fixes #8924

Type of change

[ ] Bugfix
[x] Feature

Checklist

[x] I have read the contribution guide: https://vaadin.com/docs/latest/contributing/pr
[x] I have added a description following the guideline.
[x] The issue is created in the corresponding repository and I have referenced it.
[x] I have added tests to ensure my change is effective and works as intended.
[x] New and existing tests are passing locally with my change.
[x] I have performed self-review and corrected misspellings.
[x] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.