add_threepid email confirmation emails link to /unstable/ namespace

[matrixbot]

This issue has been migrated from #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://<server>/_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://<server>/_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 matrix-org/matrix-spec-proposals#2290)