fix linter issue

Author: hellohuanlin

packages/diagrams/lib/src/container.dart

@@ -46,7 +46,7 @@ class ContainerDiagram extends StatelessWidget implements DiagramMetadata {
               child: Text('Hello World',
                   style: Theme.of(context)
                       .textTheme
-                      .headline4!
+                      .headlineMedium!
                       .copyWith(color: Colors.white)),
             ),
           ),

packages/diagrams/lib/src/curve.dart

@@ -264,7 +264,7 @@ class TranslateSampleTile extends StatelessWidget {
         Text(
           name,
           textAlign: TextAlign.center,
-          style: Theme.of(context).textTheme.bodyText1!.copyWith(
+          style: Theme.of(context).textTheme.bodyLarge!.copyWith(
                 color: Colors.black,
                 fontSize: 12.0,
               ),

packages/diagrams/lib/src/implicit_animations.dart

@@ -133,7 +133,7 @@ class AnimatedDefaultTextStyleDiagram
   @override
   Widget buildImplicitAnimation(BuildContext context, bool selected) {
     final TextStyle selectedStyle =
-        Theme.of(context).textTheme.headline4!.copyWith(
+        Theme.of(context).textTheme.headlineMedium!.copyWith(
               color: Colors.red,
               fontSize: 50.0,
               fontWeight: FontWeight.w100,

packages/diagrams/lib/src/theme_data.dart

@@ -68,7 +68,7 @@ class ThemeDataDiagram extends StatelessWidget implements DiagramMetadata {
                     secondary: Colors.green,
                   ),
                   textTheme: const TextTheme(
-                      bodyText2: TextStyle(color: Colors.purple)),
+                      bodyMedium: TextStyle(color: Colors.purple)),
                 ),
                 home: Stack(
                   children: <Widget>[