Unfortunately I do not think you can change the "noindex,nofollow" string as it comes from the page level field of Exclude from Search, as it is default behavior. It looks to be in the compiled portion of the CMS. You could choose not to use that checkbox and build your own field and webpart to render the meta tag however.
Now Robots.txt should be fully controllable by you in Kentico if you have it setup as the documentation states. https://docs.xperience.io/k12sp/configuring-kentico/search-engine-optimization/managing-robots-txt