Update GoCloud to v0.21.1+
This is needed to support Azure Managed Identities: #27303 (closed)
Note that we cannot use the GoCloud v0.21.1 tag due to a regression (https://github.com/google/go-cloud/pull/2934) that is fixed in master
(https://github.com/google/go-cloud/pull/2934).
Edited by Stan Hu