Accordion header size

### Description of the bug

When setting `headingLevel` of the `Accordion` component, the size of the header of the `Accordion` does not change. The `headingLevel` is made for specifying the correct heading level according to other heading levels on the page for accessibility reasons.

There could/should be a way of setting the size, either with the `headingLevel`-property, or with another property.

### Steps To Reproduce

1. Add an Accordion-component
2. Add a title
3. Add the `headingLevel` property
4. Observe that the title does not change size

### Additional Information

This issue was reported on Slack

Start by locating the Accordion component and its headingLevel prop, then inspect how the header size is selected. Reproduce the mismatch using the listed steps and clarify whether headingLevel or a separate property should control sizing; done means the agreed behavior is implemented and covered by the relevant component test.

Written by the indexing model from the issue text.