I think there is no best practice for this. In my past project, I created a separate directory for storing such SQL script.
For example src/main/db.
It won’t be packaged to final JAR by default (which are the preferred way in most case), yet it is convenient enough to let it packaged in assembly. You can even package them in the main artifact JAR, by adding corresponding resource declaration or using maven build-helper plugin.
However, all depends on your usage on this script. My simple “rule of thumb” is that, I would consider putting them in resources/ only when they are really resources to be loaded by the application.