I get an error downloading javax.media.jai_core:1.1.3 from maven central

The problem at this moment is that maven-central doesn't have the .jar, which is a dependency from geotoolkit

If you need it, you could use the next public repositories:

https://maven.geotoolkit.org (jai-core is here)

https://repo.osgeo.org/repository/release/

Make sure geotoolkit-repo is before Maven Central, so that it resolves before Central which misses the jar.

And move the http://download.osgeo.org/webdav/geotools repo to the first position in your repo list. Otherwise it will probably still give you that error.