High-performance RPC framework gRPC 1.40.0 released

gRPC 1.40.0 is now released, and the specific updates are as follows:

Core:

• Update Envoy API to the latest version (2021-07-30) (#26848)
• Retries are enabled by default (#26766)
• Add opentelemetry as a submodule of the latest xDS API (#26850)
• Details: https://github.com/grpc/grpc/releases/tag/v1.40.0