Add `last_used_at` field to personal_access_token API

What does this MR do and why?

Implements #328043 (closed)

Add the field last_used_at to the personal access tokens API

Screenshots or screen recordings

➜ curl http://localhost:3000/api/v4/personal_access_tokens -H "PRIVATE-TOKEN: ###" | jq

[
  {
    "id": 2,
    "name": "test_last_used",
    "revoked": false,
    "created_at": "2021-10-06T17:56:53.723Z",
    "scopes": [
      "read_api"
    ],
    "user_id": 1,
    "last_used_at": "2021-10-06T17:58:37.550Z",
    "active": true,
    "expires_at": null
  }
]

How to set up and validate locally

1. Create and use a personal access token
2. Invoke the personal access tokens endpoint

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.