Download latest when MAIN=3.1

[jodygarnett]

When fixing build server nightly, I noticed docker was failing with:

Release from branch main GeoServer 3.1-SNAPSHOT as geoserver-docker.osgeo.org/geoserver:3.1.x
Release from branch main GeoServer 3.1-SNAPSHOT (with GDAL) as geoserver-docker.osgeo.org/geoserver:3.1.x-gdal
clean: 
Downloading GeoServer for version 3.1-SNAPSHOT
  Download GeoServer 3.1-SNAPSHOT
  Download nightly build from https://build.geoserver.org/geoserver/3.1.x
  downloading geoserver-3.1.x-latest-war.zip
Build step 'Execute shell' marked build as failure

The correct download location should instead be https://build.geoserver.org/geoserver/main

This PR fixes the download.sh script to recognize that 3.1.x is main. The download.sh script is called by release.sh to avoid downloading twice (for both normal image and gdal image).

[jodygarnett]

Tested locally, since main is broken I wish to fix this for the build server.

[jodygarnett]

This was successful, the build server has now published:

geoserver-docker.osgeo.org/geoserver:3.1.x