main.star: update protobuf deps to v31.0

This is similar to previous changes like https://go.dev/cl/633420.

I updated main.star manually and then ran main.star.

Change-Id: I568bcc3ad069705b055254845d60e4fa30e08821
Reviewed-on: https://go-review.googlesource.com/c/build/+/673995
Reviewed-by: Lasse Folger <lassefolger@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
2 files changed