The [documentation][1] seems to indicate that the right maven repository to use is:

      <repository>
         <id>osgeo</id>
         <name>Open Source Geospatial Foundation Repository</name>
         <url>http://download.osgeo.org/webdav/geotools/</url>
      </repository>

However it seems that the latest version on this repository was last updated in 2014. 
https://mvnrepository.com/artifact/org.geotools.jdbc/gt-jdbc-postgis?repo=osgeo

On the other hand the Boundless one seems to be carrying a much more updated version than the other repositories. This one was last updated in September 2018:
https://mvnrepository.com/artifact/org.geotools.jdbc/gt-jdbc-postgis/20.0

However, the documentation states that the Boundless repository is for snapshots.

Which is the proper version to use?




  [1]: http://%20http://docs.geotools.org/latest/userguide/tutorial/quickstart/maven.html