Add --package-set option to spago upgrade

[fsoikin]

Description of the change

Fixes #1191

 λ npx spago upgrade --package-set 55.1.0

<snip>
Upgrading the package set to the specified version: 55.1.0


 λ npx spago upgrade --package-set 55.2.0

<snip>
Upgrading the package set to the specified version: 55.2.0


 λ npx spago upgrade --package-set 55.2.0

<snip>
✅ Nothing to upgrade, you already have the specified package set.


 λ npx spago upgrade

<snip>
Upgrading the package set to the latest version: 55.3.0


 λ npx spago upgrade

<snip>
✅ Nothing to upgrade, you already have the latest package set.


 λ npx spago upgrade --help

<snip>
  --package-set ARG        Optional package set version to be used instead of
                           the latest one

Checklist:

• Added the change to the "Unreleased" section of the changelog
• [ ] Added some example of the new feature to the README
  • It was already there: https://github.com/fsoikin/spago/blob/master/README.md/#upgrading-packages-and-the-package-set
• Added a test for the contribution (if applicable)

[f-f]

I love that it was already documented 😂
Thanks!