fix(expansion-panel): arrow not centered vertically and incorrect title font weight

[crisbeto]

• Fixes the arrow in the expansion panel header not being quite centered, due to it being based on a rotated CSS triangle. These changes switch to use an SVG which is much easier to reason about.
• Uses the correct font weight for the panel header's title.

[crisbeto]

@josephperrott can you take another look?

[ngbot[bot]]

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[crisbeto]

Closing in favor of https://github.com/angular/components/pull/28603.

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