[proposal] Release dartdoc 7.0.0, update to it in SDK DEPS, propose cherry pick update to 3.2 `beta`

[parlough]

Primarily to get the fixes for dart-lang/pub-dev#7058 and similar issues in: #3507 and #3510

These issues impact Dart 3.1, but I don't think it's worth cherry-picking back to 3.1 as the fixes would likely need to be separated from other recent dartdoc changes.

[srawlins]

We vet a release in Google internal before tagging the pub package version. So I like the idea of bumping in dart-lang/sdk, then Google internal, then bumping to 7.0.0, release, and propose a cherry-pick for Dart 3.2.

[parlough]

Sounds great to me. Thanks :)

[parlough]

I'm going to close this as the release was made and incorporated into the SDK. Thank you!

A separate cherry-pick request issue can be made against the SDK repository if you think it's safe enough.

Edit: I am going to do a bit more testing first :) - #3525