[pigeon] Update analyzer and formatter

[stuartmorgan-g]

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 flutter/flutter#161820
Fixes flutter/flutter#161583

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• 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.

[flutter-dashboard]

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[stuartmorgan-g]

text-exempt: updating dependencies

[tarrinneal]

I'm assuming you ran the generators and nothing changed?

[parlough]

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

Just a note, if you want to opt in to the new formatter, you'll need a language version of 3.7 or later, not 3.6.

[stuartmorgan-g]

I'm assuming you ran the generators and nothing changed?

In theory CI would catch that, but I'll double-check that I did run it locally before I land.

Just a note, if you want to opt in to the new formatter, you'll need a language version of 3.7 or later, not 3.6.

Ah, right, I was thinking of the wrong roller that gave us issues. I'll stick with 3.6 to match current stable.

[stuartmorgan-g]

In theory CI would catch that, but I'll double-check that I did run it locally before I land.

Verified locally. Of course, we run everything through the formatter after we generate, so I can't swear the format of the raw output didn't change. But if it did, it shouldn't matter anyway, since it's just formatted to be something other than one giant line, not for some exact output format.