`creation_ts` is seconds while `consent_ts` is milliseconds on `/_synapse/admin/v2/users/:userId`

This issue has been migrated from [#15459](https://github.com/matrix-org/synapse/issues/15459).

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### Description

See title :)

It's a minor annoyance that the fields are different units. My vote would be milliseconds.

Other endpoints may be similarly affected.

### Steps to reproduce

- Call the endpoint

### Homeserver

n/a

### Synapse Version

n/a

### Installation Method

Other (please mention below)

### Database

n/a

### Workers

Single process

### Platform

n/a

### Configuration

_No response_

### Relevant log output

```shell
n/a
```

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

_No response_

Start by tracing the /_synapse/admin/v2/users/:userId endpoint implementation and its existing tests. Confirm why creation_ts and consent_ts use different units, then make the fields consistent in milliseconds and add or update a regression test covering the response.

Written by the indexing model from the issue text.