all: release v1.21.0

Change-Id: I6a91f3efd45ab0a446b220fddc8bd03aec93b388
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/228177
Reviewed-by: Damien Neil <dneil@google.com>

all: release v1.20.1

Change-Id: I77c61b55d3c6aa540103ffe82c9dd226959bb753
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/222698
Reviewed-by: Damien Neil <dneil@google.com>

all: release v1.20.0

Change-Id: I3bc9d46f836535393059b9a73e8364211eee9e38
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221783
Reviewed-by: Herbie Ong <herbie@google.com>