chore: downgrade gapic-libraries-bom to 1.85.0 (#12808)

gapic-libraries-bom was [released with
1.85.1](#12805) due
to partial release of vector search. However, versions.txt was not
updated accordingly and caused unrelated PRs to downgrade it.

Manually downgrade it to 1.85.0 to avoid affecting other PRs.

For selective releasing in the future, we should either not release
gapic-libraries-bom or change versions.txt and release
`google-cloud-pom-parent` as well.

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Co-authored-by: cloud-java-bot <cloud-java-bot@google.com>

Author: blakeli0