Allow build 3.0.0, min language 3.8.0, generate new style, reformat

[davidmorgan]

For #394

There is no change needed to allow the new build 3.0.0 as the changes relate to using the analyzer, which graphql_codegen does not.

But it does require updating to build_test 3.0.0 which needs some small changes to tests.

And, it causes json_serializable 6.10.0 to be used, which required language version 3.8.0, so increase the minimum language version. This triggers a huge reformat(!) because the dartfmt style changes at language version 3.7.0. The second commit is just the reformat from dart format ..

[davidmorgan]

Oh ... this doesn't work yet due to the effect of the reformat on golden file compares ... will see if I can figure out a good fix.

[davidmorgan]

Changing the generated output to also be 3.8.0 style seems to have done the trick.

That should be what people want, since the language version of the package is now 3.8.0 so they should be on that style.