Prepare v0.6.7 for publishing

[mit-mit]

Prepare for publishing 0.6.7

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[csells]

@mit-mit thoughts about flipping the version to 1.0?

[mit-mit]

I think it might be better to get some more feedback first, so that we can still change APIs etc. without needing new major versions?

[csells]

If we want to support new Gemini 2 features, we'll need to bump the version and change the API. Whether you want it to be from 0.x or from 1.x is up to you.

[Piinks]

LGTM