Cleanup IAM policy, update webauthn Go library

Changed (non-breaking)

• Replace deprecated duo-labs/webauthn with go-webauthn/webauthn.

Fixed

• Whitespace formatting using go fmt and gofumpt.
• Remove the duplicate "..._global" resource from the Lambda Role's policy
  • The api_key_table and webauthn_table values now include the full table name, so this transitional additional resource name (where we manually appended the "_global" suffix) is no longer needed.