add_threepid email confirmation emails link to /unstable/ namespace

This issue has been migrated from [#9197](https://github.com/matrix-org/synapse/issues/9197).

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When you add an email address to an account, you get an email which invites you to click on a link whose path is `https:///_matrix/client/unstable/add_threepid/email/submit_token?token=...&client_secret=...`.

We should not be using an `unstable` prefix for stable features. The correct path for this would be something under `https:///_synapse/client/...`. Of course we'll have to support both for a while to avoid breaking the links in any emails that have already been sent.

(This was added in #6042, and probably should have been cleaned up as part of finalising https://github.com/matrix-org/matrix-doc/pull/2290)