Update Prettier formatter in tests

[verdverm]

Situation:

• The prettier SDK changed the API between v2 & 3.
• we hard code the formatter version in the general GHA workflow, so we don't use the one built on the current commit (we don't often run the action that builds formatters because they are so flakey...)

To upgrade, we need to

• update the formatter and publish a new version, expecting a build failure in the release tag build
• update the pinned version to v0.6.10-rc.2 for the v0.6.10 release
• update the pinned version to v0.6.10 after the release has been cut