Release v18.0.0

[BinaryMuse]

🔨 Breaking changes

Some breaking changes that were meant to be included in v17.0.0 were mistakenly absent. Those changes are included in this release, and the v17.0.0 release notes have been edited to reflect the changes included in that release. Big thanks to @acknosyn for catching and fixing this. (#757, #758)

• Dialog components no longer render with a header by default. Extract Dialog header into separate component #736 @BinaryMuse

  Migration strategy: change any Dialog component with a title prop set as follows:

  <Dialog title="...title...">
    ...content...
  </Dialog>

  to

  <Dialog>
    <Dialog.Header>...title...</Dialog.Header>
    ...content...
  </Dialog>

• Heading now renders an h2 by default instead of an h1 Heading update (new default level and updated docs) #716 @taylorcjohnson

  Migration strategy: change any use of the Heading component without an as prop:

  <Heading>...</Heading>

  to

  <Heading as="h1">...</Heading>

🐛 Bug Fixes

• Update box shadow to match Primer CSS Next Update box shadow to match Primer CSS Next #732 @colebemis

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