Use `go 1.20` in go.mod #2668

Job	Run time
staticcheck	1m 26s
lint	24s
build	1m 1s
tests (macos/go-1.20-amd64)	2m 35s
tests (ubuntu/go-1.20-386)	1m 10s
tests (ubuntu/go-1.20-amd64)	1m 31s
tests (windows/go-1.20-386)	2m 39s
tests (windows/go-1.20-amd64)	4m 3s
tests (macos/go-1.21-amd64)	1m 48s
tests (ubuntu/go-1.21-386)	1m 12s
tests (ubuntu/go-1.21-amd64)	1m 27s
tests (windows/go-1.21-386)	2m 2s
tests (windows/go-1.21-amd64)	4m 16s
test-python-example	17s
test-gsutil-example	53s
test-node-example	7s
test-go-example	50s
test-dotnet-example	18s
test-scala-example	1m 1s
test-java-example	35s
test-cpp-example	3m 55s
test-curl-example	5s
test-curl-both-scheme-default-ports-example	9s
test-curl-both-scheme-non-default-ports-example	5s
unblock-pr	2s
	33m 51s

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