v21.0.0

This release does not change the pinned API version. It's still 2026-06-24.dahlia.

We're releasing it as a major out of an abundance of caution, but it should be functionally a patch release for most users. See below.

• #2097 ⚠️ Correctly type properties on ErrorObject
  • the properties of ErrorObject were typed as string when many of them should have been null|string. If you (or your typechecker) were treating these as plain strings, you'll need to be more defensive in your code.
  • to be clear: no runtime code has changed, we've just made the types more accurate. We didn't want to break any builds in a patch version, so this is released as a major
• #2098 Replace source hash with Telemetry UUID
• #2095 Remove unused Retry-After header support

See the changelog for more details.