[grass7][mac] Use the most recent GRASS version

This is backporting: #9200
It is a continuation of:

    * [8db3dea](8db3dea)
    * [9efe4c5](9efe4c5)

This commit only affects users that have multiple GRASS installations on
their Macs. Using the most recent GRASS version is what we do on Linux too.

Author: pmav99

python/plugins/processing/algs/grass7/Grass7Utils.py

@@ -223,7 +223,7 @@ def grassPath():
                         break
                     # If nothing found, try standalone GRASS installation
                     if folder is None:
-                        for version in ['0', '1', '2', '4']:
+                        for version in ['6', '4', '2', '1', '0']:
                             testfolder = '/Applications/GRASS-7.{}.app/Contents/MacOS'.format(version)
                             if os.path.isdir(testfolder):
                                 folder = testfolder