Bump k6 version to fix ci build

[pablochacin]

Since package go.buf.build/grpc/go/prometheus/prometheus is no longer available (it was deprecated and then removed) building a custom k6 binary with the disruptor fails with the following error:

go.k6.io/k6/cmd imports
	github.com/grafana/xk6-output-prometheus-remote/pkg/remotewrite imports
	go.buf.build/grpc/go/prometheus/prometheus: unrecognized import path "go.buf.build/grpc/go/prometheus/prometheus": https fetch: Get "https://go.buf.build/grpc/go/prometheus/prometheus?go-get=1": dial tcp: lookup go.buf.build: no such host
2023/08/08 19:04:51 [INFO] Cleaning up

k6 v0.45.1` updates its dependencies to fix this issue.