OOMMF: set preferred version (#35675)

With the last merge request for OOMMF [1], the intention was to have version
20b0_20220930 as the preferred version, and provide 20b0_20220930-vanilla as an
additional version for the unlikely case anybody needed that.

I made the (wrong) assumption that the `version` listed first in the `package.py` file
would be the preferred version. This merge request is to correct that by
explicitly tagging the preferred version with `preferred=True`.

[1] https://github.com/spack/spack/pull/33072/files

var/spack/repos/builtin/packages/oommf/package.py

@@ -49,7 +49,9 @@ class Oommf(Package):
     maintainers("fangohr")
 
     version(
-        "20b0_20220930", sha256="764f1983d858fbad4bae34c720b217940ce56f745647ba94ec74de4b185f1328"
+        "20b0_20220930",
+        sha256="764f1983d858fbad4bae34c720b217940ce56f745647ba94ec74de4b185f1328",
+        preferred=True,
     )
 
     version(