fix logging to use enum instead of int

[Nimrod51]

No description provided.

[kannes]

This PR is a fix for QGIS4/Qt6 compatibility. Otherwise the logging would crash:

iface.messageBar().pushMessage(title="foo", text="bar", level=1)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: QgsMessageBar.pushMessage(): arguments did not match any overloaded call:
  overload 1: argument 'level' has unexpected type 'int'
  overload 2: argument 'level' has unexpected type 'int'
  overload 3: 'level' is not a valid keyword argument

while iface.messageBar().pushMessage(title="foo", text="bar", level=Qgis.MessageLevel.Warning) works.