delete_stale_devices_after pruning MSC3202 AS devices

### Description

With MSC3202 AS' can use devices for E2EE. However, the AS users devices `devices.last_seen` entry is never updated past the initial creation date. When using `delete_stale_devices_after` or out of band cleanup scripts, they will eventually get pruned and E2EE fails.

MSC3202 notes: "To compliment the (optional) user_id when using an as_token as an access token, a similarly optional device_id query parameter is proposed. When provided, the server asserts that the device ID is valid for the user, and that the appservice is able to masquerade as that user. If valid, that device ID should be assumed as being used for that request. **For many requests, this means updating the "last seen IP" and "last seen timestamp" for the device** [...]"

Query to list affected devices
`select user_id, device_id, to_timestamp(last_seen / 1000) last_seen from devices where hidden = false and to_timestamp(last_seen / 1000) < now() - interval '';`

### Steps to reproduce

- set following config options
```
delete_stale_devices_after = 180d
experimental_features:
msc3202_device_masquerading: true
msc3202_transaction_extensions: true
```
- use an AS with MSC3202 support, like matrix-hookshot
- AS creates devices for its users, with devices.last_seen set to the creation date, but is never updated
- E2EE breaks for AS users after time configured in `delete_stale_devices_after`

### Homeserver

localhost

### Synapse Version

1.122.0

### Installation Method

Debian packages from packages.matrix.org

### Database

PostgreSQL 13

### Workers

Multiple workers

### Platform

Debian 11

### Configuration

_No response_

### Relevant log output

```shell
---
```

### Anything else that would be useful to know?

_No response_