Release v1.21.0

This release updates the repackaged OpenTelemetry Java Agent to 1.21.0.

Check out the release notes for upstream versions

• https://github.com/open-telemetry/opentelemetry-java/releases/tag/v1.21.0
• https://github.com/open-telemetry/opentelemetry-java-contrib/releases/tag/v1.21.0
• https://github.com/open-telemetry/opentelemetry-java-instrumentation/releases/tag/v1.21.0