cl/504661697 removes this build tag once and for all, but it is blocked on
additional work needed to get it working with OSS bazel builds.
Until that is ready, bump the build tag to unblock testing Go tip (1.21).
PiperOrigin-RevId: 505923005
This package provides additional synchronization primitives not provided by the
Go stdlib 'sync' package. It is partially derived from the upstream 'sync'
package from go1.10.