feat: enable UI by default (#8245)

### Description

TSIA

### Testing Instructions

~~Unit test update shows that default is now true~~ I forgot that these
tests don't run in a TTY so they still won't have the UI enabled.

Manual test that a fresh repo with no `experimentalUI` key in
`turbo.json` will use the UI.

crates/turborepo-lib/src/config.rs

@@ -268,7 +268,7 @@ impl ConfigurationOptions {
     }
 
     pub fn experimental_ui(&self) -> bool {
-        self.experimental_ui.unwrap_or_default() && atty::is(atty::Stream::Stdout)
+        self.experimental_ui.unwrap_or(true) && atty::is(atty::Stream::Stdout)
     }
 }