fix: update the SQLAlchemy model definition at json column for Log ta…

…ble (apache#25445)

superset/models/core.py

@@ -987,7 +987,7 @@ class Log(Model):  # pylint: disable=too-few-public-methods
     user_id = Column(Integer, ForeignKey("ab_user.id"))
     dashboard_id = Column(Integer)
     slice_id = Column(Integer)
-    json = Column(Text)
+    json = Column(utils.MediumText())
     user = relationship(
         security_manager.user_model, backref="logs", foreign_keys=[user_id]
     )