The "timezone" plugin does not parse the timestamp in seconds

**Describe the bug**
The "timezone" plugin does not parse the timestamp in seconds
Example:
`dayjs.tz(1690278073, 'X', "Europe/London").format(); // => "1970-01-20T14:31:18+01:00"`

**Expected behavior**
`dayjs.tz(1690278073, 'X', "Europe/London").format(); // => ""2023-07-25T10:41:13+01:00"`

**Information**
- Day.js Version [v1.11.9]
- OS: [Ubuntu 20.04.6]
- Browser [chrome 115.0.5790.102]
- Time zone: [GMT+03:00]

Start with the timezone plugin and reproduce the reported call using the Day.js v1.11.9 setup. Check how the plugin handles the 'X' format and compare its output with the expected 2023 timestamp; done means the example formats correctly without regressing other timezone parsing.

Written by the indexing model from the issue text.