Correct the name of the `spine-gcloud-testutil` artifact in the `README.md`

[MykytaPimonovTD]

The README currently references an outdated artifact. It should be updated from io.spine.gcloud:testutil-gcloud:1.9.1 to io.spine.gcloud:spine-testutil-gcloud:1.9.1.

[alexander-yevsyukov]

We should use the TestLib name for the artifact as we conventionally use in other projects. After that the link in README.md should be updated accordingly.