Remove "colon specifier is experimental feature (issue #5660)" for 3.8

[phadej]

Blocked on #5660

[phadej]

Re-milestoning this for 3.6, as it's blocked on #5660.

[gbaz]

What even is this about? :-)

[jneira]

cabal/Cabal/src/Distribution/Types/Mixin.hs

Line 66 in 00a2351

parsecWarning PWTExperimental "colon specifier is experimental feature (issue #5660)"

cabal/Cabal/src/Distribution/Types/Dependency.hs

Line 144 in 00a2351

parsecWarning PWTExperimental "colon specifier is experimental feature (issue #5660)"

???