Upgrade express to 4.21.2 (#90)

## Motivation for the change, related issues

I propose upgrading the express package to 4.21.2 to fix a few security
vulnerabilities in the express and its dependencies.

## Implementation details

I did the following:
1. Bumped version in `package.json`
2. Ran `npm install`

## Testing Instructions (or ideally a Blueprint)

- Start the Playground CLI `bun packages/playground/cli/src/cli.ts
server`
- Confirm that WordPress loads and static assets are loaded
- Go to WP-admin to ensure users can log in 
- Open a post page to confirm that pretty permalinks work

Author: wojtekn