Fix a few readme links regarding ESS (#19070)

Didn't add a changelog, is it needed for this?

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.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.
* [ ] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

README.rst

@@ -59,7 +59,7 @@ below for more useful documentation links.
 Enterprise quality support for Synapse including SLAs is available as part of an
 `Element Server Suite (ESS) <https://element.io/pricing>`_ subscription.
 
-If you are an existing ESS subscriber then you can raise a `support request <https://ems.element.io/support>`_
+If you are an existing ESS subscriber then you can raise a `support request <https://customer.element.io/support>`_
 and access the `Element product documentation <https://docs.element.io>`_.
 
 🤝 Community support

changelog.d/19070.doc

@@ -0,0 +1 @@
+Update ESS support link to point to the new customer portal domain.