prepare RC2

i did a composer update and suggest that we add the prefer-stable to get better 3rd party versions.

we need not forget to change to minimum-stability stable once we release the bundles.

i also removed the UPGRADE files as they are about symfony-standard and not about the cmf.

should we compile our own UPGRADE file from the bundle upgrades? it seems to me the UPGRADE in here is not about changes in dependencies but only of the standard edition. so maybe explain how to go from SE 1.0 to this SE?