Make version optional

[nicodes]

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

[KSXGitHub]

If version is not specified, packageManager field in package.json would be required. If both don't exist, there would be error.

The reason it was made an error was because we'd like to have the freedom of making breaking changes without fear of breaking existing CIs.