Bump go.opentelemetry.io/otel/sdk from 1.20.0 to 1.21.0 in /extension/zpagesextension #843

Job	Run time
govulncheck	1m 15s
setup-environment	30s
lint	1m 22s
checks	26s
test-coverage	1m 19s
cross-build-collector (darwin, amd64)	8s
unittest-matrix (~1.21.4)	1m 13s
cross-build-collector (darwin, arm64)	9s
unittest-matrix (~1.20.11)	1m 14s
cross-build-collector (linux, 386)	17s
cross-build-collector (linux, amd64)	11s
cross-build-collector (linux, arm64)	11s
cross-build-collector (linux, ppc64le)	7s
cross-build-collector (linux, arm, 7)	10s
cross-build-collector (linux, s390x)	11s
cross-build-collector (windows, 386)	8s
cross-build-collector (windows, amd64)	7s
unittest	1s
	8m 59s

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