chore(deps): bump source_gen in /packages/luthor_generator

Bumps [source_gen](https://github.com/dart-lang/source_gen) from 1.5.0 to 2.0.0.
- [Release notes](https://github.com/dart-lang/source_gen/releases)
- [Commits](dart-lang/source_gen@source_gen-v1.5.0...source_gen-v2.0.0)

---
updated-dependencies:
- dependency-name: source_gen
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

packages/luthor_generator/pubspec.yaml

@@ -15,7 +15,7 @@ dependencies:
   freezed_annotation: ^2.4.3
   json_annotation: ^4.8.1
   luthor: ^0.5.2+1
-  source_gen: ^1.5.0
+  source_gen: ">=1.5.0 <3.0.0"
 
 dev_dependencies:
   lint: ^2.3.0