[EmptyState] Title font-size should not depend on it's semantic level

[BertrandPodevin]

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EmptyState title font-size (and line-height) is currently tied to it's semantic level (h1, h2, h3...).

It should not be the case : the font-size should always be --sizes-L-fontSize or var(--sizes-XXL-fontSize) and carried by the class emptyState-content-heading, no matter what semantic level the title realy is in the DOM.

It also seems that the Angular component for the emptyState always use h1 or h3 for it's title level and do not provide an option to change this. In many cases it won't be the correct semantic title level to use.