fix(overlays): focus trap refs cleared on dismiss

[liamdebeasi]

Pull request checklist

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
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Pull request type

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• Bugfix
• Feature
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• Refactoring (no functional changes, no api changes)
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What is the current behavior?

Part of #26833

Our focus trapping util stores the last element that was focused in overlayEl.lastFocus. However, this is never cleared when the overlay is dismissed. This results in the overlay keeping a strong reference to some other element. lastFocus is only utilized while the overlay is presented, so we should clear it on dismiss.

What is the new behavior?

• lastFocus is cleared when the overlay is dismissed.

Does this introduce a breaking change?

• Yes
• No

Other information

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[liamdebeasi]

This should always have been HTMLIonOverlayElement. Without it, I get typescript errors when trying to access lastFocus (since it is only known property of HTMLIonOverlayElement not HTMLElement)