Update asciidoctor-plantuml gem to v0.0.12

This adds support for the deflate encoding:
https://github.com/hsanson/asciidoctor-plantuml/pull/12

Currently this can only be changed by setting the `PLANTUML_ENCODING`
environment variable to `deflate`.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/216587