Update Matrix notification integration

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What does this MR do and why?

• Switched from the ?access_token query param to the Authorization header, as the query param was deprecated in Matrix v1.11: https://spec.matrix.org/v1.13/client-server-api/#using-access-tokens (and it will be removed at some point in the future: https://github.com/matrix-org/matrix-spec-proposals/pull/4127)
• Changed the default server URL from https://matrix.org to https://matrix-client.matrix.org as per the .well-known file: https://matrix.org/.well-known/matrix/client
• Changed the message type for notifications from m.text to m.notice. m.notice is otherwise equivalent to m.text, but it's meant for automated messages: https://spec.matrix.org/v1.13/client-server-api/#mnotice

MR acceptance checklist

Screenshots or screen recordings

How to set up and validate locally

1. Checkout the branch
2. Verify that the example server address is https://matrix-client.matrix.org
3. Configure Matrix integration
4. Click on Test Settings button
5. Verify that the message is sent successfully and uses the m.notice message type.

   • Element Web/Desktop (https://app.element.io) renders m.notice messages with a slightly lighter font color: