[java-bigtable] Clean up docfx generation

It looks like there's still some configuration for documentation generation to googleapis.dev here: https://github.com/googleapis/java-bigtable/blob/main/pom.xml#L247

Since googleapis.dev has been redirected to cloud.google.com, this can probably be removed.

Separately, it looks like bigtable is still using v1.5.0 of the doclet: https://github.com/googleapis/java-bigtable/blob/main/pom.xml#L325, even though the latest is 1.8.0 (https://github.com/googleapis/java-docfx-doclet/releases/tag/1.8.0). Not sure if this was intentionally kept at the earlier version or if it was just missed in the update to 1.8.0?