Remove main branch and make 6.0 the default

[mhsdesign]

i wonder if we have to adjust any of these requirements then

https://github.com/neos/neos-development-distribution/blob/49544fbe19d25855f43231d4ea59abf7f1246f03/composer.json#L39

https://github.com/neos/neos-setup/blob/63417aa6cd17351609d982f1f20befd67f425d0c/composer.json#L8

but the idea is to reduce confusion what is the current branch to just branch first a new version and have no random main branch see: https://github.com/neos/neos-setup/blob/63417aa6cd17351609d982f1f20befd67f425d0c/composer.json#L8

[kitsunet]

I don't see either as a problem, we should probably fix neos/neos-setup to remove the non existing branch then... The distribution essentials says "no version recquirement" and it can even be an untagged version, that seems fine.