taskgroup: mark NoError as deprecated

Remove the last remaining use in the package, and add a deprecation annotation
so staticcheck et al. will complain about it.

taskgroup.go

@@ -179,6 +179,8 @@ func adaptErrorFunc(ef any) errorFunc {
 }
 
 // NoError adapts f to a Task that executes f and reports a nil error.
+//
+// Deprecated: Use [Group.Run] or [taskgroup.Run] instead.
 func NoError(f func()) Task { return func() error { f(); return nil } }
 
 func noError(f func()) Task { return func() error { f(); return nil } }

throttle.go

@@ -54,4 +54,4 @@ func (s StartFunc) Go(task Task) { s(task) }
 
 // Run is a legibility shorthand for calling s with a task that runs f and
 // reports a nil error.
-func (s StartFunc) Run(f func()) { s(NoError(f)) }
+func (s StartFunc) Run(f func()) { s(noError(f)) }