Bump master branch version number to 0.7.0.dev0 (#1254)

I have been forgetting to increment our master branch release number,
so it's now less than our pip release :)

I think we can consider the master branch a preview of our 0.7.0
release at this point.

keras_nlp/__init__.py

@@ -28,4 +28,4 @@
 from keras_nlp import utils
 
 # This is the global source of truth for the version number.
-__version__ = "0.6.0.dev0"
+__version__ = "0.7.0.dev0"