Adjust UI to align with documenation (labels)

[mortensnedker]

Getting rid of the term 'secondary' in relation to a build, please adjust the UI of the scheduled task add-in, switching 'secondary' to 'alternate':

Documenation:
https://doc.dynamicweb.dev/manual/dynamicweb10/settings/system/system/scheduledtaskaddins/build-repository-index.html