Chaging the default cabal spec in cabal init from 3.0 to 3.4

I'd like to suggest that we update the default spec that is suggested to the user upon `cabal init`.

Currently the user is offered

> * 5) 3.0 (+ set notation for ==, common stanzas in ifs, more redundant commas, better pkgconfig-depends)

by default, but has to manually input

> 6) 3.4 (+ sublibraries in 'mixins', optional 'default-language')

in order to enjoy the same things + the additions of 3.4.