[EuiAccordion][AXE-CORE]: Interactive content in trigger must not have aria-expanded attribute

[1Copenut]

Description

@darnautov brought this issue to our attention with Kibana PR 154237.

The EuiAccordion allows users to add interactive content next to the expand/collapse button as a way to surface multiple controls. When we do this, axe-core is throwing an error because we have an aria-expanded="true | false" attribute on a non-button element.

I'd recommend we check the buttonElement prop against the accepted values and conditionally render the aria-expanded attribute on accordion.tsx#L377 only if the ButtonElement === 'button'.

WCAG guidance

• [SC 4.1.2: Name, Role, Value (Level A)])(https://www.w3.org/WAI/WCAG21/Understanding/name-role-value.html)

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