Adds compatibility with the latest major version of `analyzer` and `dart_style` (the latter being required to actually allow `analyzer` 7).

The breaking change of `dart_style` does affect our usage, so this updates rather than expands the allowed version. The range for `analyzer` is updated to drop versions older thas 6 since `dart_style` doesn't allow versions older than 6 anyway.

We have to pick a language version to use for formatting; this (somewhat arbitrarily) uses 3.6, to opt in to the new formatter.

Fixes https://github.com/flutter/flutter/issues/161820
Fixes https://github.com/flutter/flutter/issues/161583