go.mod: bump version to go 1.21

Also run `go mod tidy`.

Change-Id: Ia0402b5a282ebaa6ec1fc8d75463bf1295573484
Cq-Include-Trybots: luci.golang.try:x_benchmarks-gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/599715
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
2 files changed