Deprecate variadic version of `+=` operator on `Growable`

[julienrf]

We want to prevent users from using it so that the transition to non-auto-tupling will be easier.

See related discussion here: scala/scala3#4311 (comment)

[lrytz]

Also remove the overload in LongMap and add an add a final override, as suggested by Sébastien scala/scala3#4311 (comment). The scala inliner handles it (and eliminates the boxing and tuple allocation), but of course only if it's enabled (collection users need to enable -opt:l:inline '-opt-inline-from:scala.collection.**')