Add deprecation notice to cranelift_use_egraphs option.

[cfallin]

After #5587, this is on by default. We are retaining the traditional (no-egraphs) path for now, selected by setting this option to false, but we eventually plan to delete it assuming that we don't find serious regressions or issues. This PR adds a deprecation notice to the option.

Updated as per this comment (thanks!).

[elliottt]

🎉

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