cmd/bent: lower default go.mod version to 1.25

I didn't realize we were still running 1.25 perf tests.
Oops.

Fixes golang/go#80674
Fixes golang/go#80675

Change-Id: I0cce967ae90eda45fc17e579433bd5c272d8a5ae
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/816920
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>