cmd: update golang.org/x/mod to v0.20.0

cmd: update golang.org/x/mod to v0.20.0

Commands run:
    go get golang.org/x/mod@v0.20.0
    go mod tidy
    go mod vendor

This is being done in preparation for vendoring in the latest commit of
golang.org/x/telemetry.

Change-Id: I67de7c23c4949cde63826c9d791f7b67698491d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/603897
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>