util/tracing: switch to semconv v0.21.0 #5234

Job	Run time
build (windows/amd64)	1m 42s
build (freebsd/amd64)	1m 41s
test-freebsd-amd64	2m 29s
test-windows-amd64 (containerd, ./client)	4m 12s
test-windows-amd64 (containerd, ./cmd/buildctl)	1m 47s
test-windows-amd64 (containerd, ./worker/containerd)	1m 57s
test-windows-amd64 (containerd, ./solver)	3m 31s
test-windows-amd64 (containerd, ./frontend)	1m 40s
test-windows-amd64 (containerd, ./frontend/dockerfile)	37m 8s
test-windows-amd64 (./..., 1)	11m 47s
	1h 7m 54s

Provide feedback