Update schema.xml in-place

[Kris-LIBIS]

With PR #7865 in place, the updateSchemaMDB.sh script no longer works as intended. It used to update the Solr schema by updating two XML files that were included in the schema.xml. Issue #7864 indicated that this update mechanism did not work and after the fix, the schema.xml file should be updated manually.

In order to restore the fully 'automatic' update without breaking #7864, the updateSchemaMDB.sh script should modify the schema.xml directly instead of updating the included XML files.