Auto merge of #127176 - fee1-dead-contrib:fx-requires-next-solver, r=compiler-errors

Make `feature(effects)` require `-Znext-solver`

Per https://github.com/rust-lang/rust/pull/120639#pullrequestreview-2144804638

I made this a hard error because otherwise it should be a lint and that seemed more complicated. Not sure if this is the best place to put the error though.

r? project-const-traits