Fix merge conflicts in PR #1658

[emlun]

This would merge into PR #1658, not the main branch.

In writing https://github.com/w3c/webauthn/pull/1663#pullrequestreview-1057229775 I had already done most of the work of working out what the merge conflicts are and how to resolve them, so I figured I might as well submit a pull request.

Summary of changes:

• Move affected steps from §5.1.4. Use an Existing Credential to Make an Assertion to the new §5.1.4.2. Issuing a Credential Request to an Authenticator section (see #1576)

  Where to see this change: diff from main to this PR

• Add steps for computing |enterpriseAttestationPossible| and |attestationFormats| to §5.1.4.2. Issuing a Credential Request to an Authenticator

  Where to see this change: diff from main to this PR

• Fix heading level of devicePubKey section (parallel of #1781)

  Where to see this change: diff from PR #1658 to this PR

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