Bump lower bound of built_redux dependency

+ This wide range was causing pub get to take 5-6 minutes.
+ All internal Workiva consumers are already on ^7.0.0

pubspec.yaml

@@ -10,7 +10,7 @@ environment:
 dependencies:
   analyzer: ">=0.30.0 <=0.31.0"
   barback: ">=0.15.2 <=0.15.2+14"
-  built_redux: ">=6.1.0 <8.0.0"
+  built_redux: ^7.4.1
   built_value: ">=4.2.0 <6.0.0"
   js: ^0.6.0
   logging: ">=0.11.3+1 <1.0.0"