"Last updated" metadata not preserved after versioning

## 🐛 Bug Report

### Description

All versioned docs have the same "last updated" info: the time and author when that version was frozen.

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues)?

Yes

## Steps to reproduce

Check https://docusaurus.io/docs/next/markdown-features/admonitions versus https://docusaurus.io/docs/markdown-features/admonitions: the former has a date that's prior to the latter because the latter is when the `2.0.0-beta.6` folder was created, not the actual doc commit date.

I guess we should freeze the last updated metadata as well? Versioned docs can still be updated though...