Sync notifications

**Description:**

I use matrix clients on my laptop and my phone. When I receive a message, notification is sent to both devices.

If I read the message on my laptop, the notification on my phone remains, although the messages are now read.
Since ntfy supports deleting notifications as of version 2.16 (see https://docs.ntfy.sh/releases/#ntfy-server-v2160), it should be possible for Synapse server to send a notification deletion when a message is read.

Start by locating Synapse's existing ntfy notification handling and the path that processes a message becoming read, then compare them with ntfy server 2.16 deletion support described in the linked release notes. Done means reading a message on one Matrix client causes the corresponding notification on another device to be deleted, with coverage for the relevant flow.

Written by the indexing model from the issue text.