Enable login_via_existing_session by default

This issue has been migrated from [#15719](https://github.com/matrix-org/synapse/pull/15719).

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Following discussion in https://github.com/matrix-org/synapse/pull/15388#pullrequestreview-1454194641 this PR enables the login_via_existing_session capability from MSC3882 by default.

As UIA is required by default and there is a rate limit of 1 request per minute on the new endpoint, I believe this qualifies as "secure by default".

### Pull Request Checklist

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog file](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#changelog). The entry should:
- Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
- Use markdown where necessary, mostly for `code blocks`.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
* [x] Pull request includes a [sign off](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#sign-off)
* [x] [Code style](https://matrix-org.github.io/synapse/latest/code_style.html) is correct
(run the [linters](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))