Make `version` optional

I think the mandatory `version` should be made optional and default to the latest stable. This will lower the entry barrier for first time users, and can cause confusion with the action version vs `pnpm` version.

Current:
```
- uses: pnpm/action-setup@v2
        with:
          version: 7
```

Proposed:
```
- uses: pnpm/action-setup@v2
```