Use Go 1.21rc4 in CI #2643

Job	Run time
staticcheck	1m 21s
build	53s
tests (macos/go-1.19-amd64)	6m 41s
tests (ubuntu/go-1.19-386)	1m 0s
tests (ubuntu/go-1.19-amd64)	33s
tests (windows/go-1.19-386)	2m 9s
tests (windows/go-1.19-amd64)	3m 6s
tests (macos/go-1.20-amd64)	1m 19s
tests (ubuntu/go-1.20-386)	19s
tests (ubuntu/go-1.20-amd64)	1m 11s
tests (windows/go-1.20-386)	3m 52s
tests (windows/go-1.20-amd64)	4m 8s
tests (macos/go-1.21.0-rc.4-amd64)	1m 58s
tests (ubuntu/go-1.21.0-rc.4-386)	1m 12s
tests (ubuntu/go-1.21.0-rc.4-amd64)	1m 21s
tests (windows/go-1.21.0-rc.4-386)	4m 37s
tests (windows/go-1.21.0-rc.4-amd64)	3m 8s
lint	23s
test-python-example	30s
test-gsutil-example	53s
test-node-example	12s
test-go-example	51s
test-dotnet-example	16s
test-scala-example	48s
test-java-example	30s
test-cpp-example	3m 41s
test-curl-example	7s
test-curl-both-scheme-default-ports-example	8s
test-curl-both-scheme-non-default-ports-example	7s
unblock-pr	1s
	47m 15s

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