Update go.mod and vendor

vendor/github.com/moby/buildkit/util/tracing/multispan.go

@@ -1,22 +0,0 @@
-package tracing
-
-import (
-	opentracing "github.com/opentracing/opentracing-go"
-)
-
-// MultiSpan allows shared tracing to multiple spans.
-// TODO: This is a temporary solution and doesn't really support shared tracing yet. Instead the first always wins.
-
-type MultiSpan struct {
-	opentracing.Span
-}
-
-func NewMultiSpan() *MultiSpan {
-	return &MultiSpan{}
-}
-
-func (ms *MultiSpan) Add(s opentracing.Span) {
-	if ms.Span == nil {
-		ms.Span = s
-	}
-}