java/netbeans: Update to 12.6

For the replacing of the python command, use the default python 3 version instead of hardcoding python 2.7.
Drop the netbeans version from the .desktop file.
Don't use absolute path for the symlink and add a non-versioned.

This also makes netbeans fetchable again, all distfiles prior to 12.6 where removed upsteam [1].

PR: ports/260414 [1]
Reported by: Marko Cupać <marko.cupac@mimar.rs> [1]