Migrate to Buf remote packages

[pkwarren]

Buf no longer supports the remote generation alpha (Go packages starting with go.buf.build). There are steps showing how to migrate to the new supported mechanism here: https://buf.build/docs/migration-guides/migrate-remote-generation-alpha/#go-module-proxy.

I'm happy to open a PR to migrate this project to use the new dependency. This will have the benefit of keeping up with future google protobuf-go releases in the future.

Reference:

xk6-output-prometheus-remote/go.mod

Line 12 in 34469a7

go.buf.build/grpc/go/prometheus/prometheus v1.4.4