Reduce SDK constraint to 2.14.0

Author: parlough

pubspec.yaml

@@ -4,7 +4,7 @@ version: 4.1.0
 description: A non-interactive HTML documentation generator for Dart source code.
 homepage: https://github.com/dart-lang/dartdoc
 environment:
-  sdk: '>=2.15.0 <3.0.0'
+  sdk: '>=2.14.0 <3.0.0'
 
 dependencies:
   analyzer: ^2.7.0