[null-safety] try updating snippets for null safety (#68723)

Add a language version so the snippets do not opt-in automatically.

Fixes #68719

dev/snippets/config/templates/stateful_widget.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_cupertino.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_cupertino_page_scaffold.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_cupertino_ticker.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_material.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_material_ticker.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_restoration.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_scaffold.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_scaffold_center.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_scaffold_center_freeform_state.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateful_widget_ticker.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateless_widget.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateless_widget_cupertino.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateless_widget_cupertino_page_scaffold.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateless_widget_material.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateless_widget_scaffold.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}
 

dev/snippets/config/templates/stateless_widget_scaffold_center.tmpl

 /// Flutter code sample for {{element}}
+// @dart = 2.9
 
 {{description}}