Deprecate `onMount` the way `beforeUpdate` and `afterUpdate` were

Svelte 5 deprecated beforeUpdate and afterUpdate. onMount should go to. It is implemented as an effect that runs its callback inside untrack:

user_effect(() => {
	const cleanup = untrack(fn);
	if (typeof cleanup === 'function') return cleanup;
});

We already say "onMount is just effect untracked" (#16556), so why not just show it that way. In a runes component onMount(fn) and $effect(() => untrack(fn)) are the same thing, and keeping two sanctioned spellings of one behavior means the docs teach a lifecycle model the rest of Svelte 5 abandoned.

The precedent already exists. beforeUpdate/afterUpdate stayed available in legacy mode and became errors in runes mode. onMount can follow the same path.