minor #11026 [Maintenance] Mention about SonataCore removal in UPGRADE.md (lchrusciel)
minor #11026 [Maintenance] Mention about SonataCore removal in UPGRADE.md (lchrusciel)

This PR was merged into the 1.7-dev branch.

Discussion
----------

| Q               | A
| --------------- | -----
| Branch?         | master
| Bug fix?        | yes
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | 
| License         | MIT

Ref. https://github.com/Sylius/SyliusDemo/pull/119

Commits
-------

ab67c8ff490ccbaea49a6a5cb0f47d67e0824b97 [Maintenance] Mention about SonataCore removal in UPGRADE.md

SyliusBot/Sylius,lchrusciel/Sylius,GSadee/Sylius,101medialab/Sylius,GSadee/Sylius,foobarflies/Sylius,mbabker/Sylius,pamil/Sylius,mbabker/Sylius,loic425/Sylius,antonioperic/Sylius,pjedrzejewski/Sylius,loic425/Sylius,lchrusciel/Sylius,antonioperic/Sylius,lchrusciel/Sylius,mbabker/Sylius,SyliusBot/Sylius,Sylius/Sylius,diimpp/Sylius,diimpp/Sylius,foobarflies/Sylius,Arminek/Sylius,pjedrzejewski/Sylius,GSadee/Sylius,Sylius/Sylius,SyliusBot/Sylius,101medialab/Sylius,pjedrzejewski/Sylius,Zales0123/Sylius,diimpp/Sylius,kayue/Sylius,Arminek/Sylius,Zales0123/Sylius,pjedrzejewski/Sylius,Arminek/Sylius,foobarflies/Sylius,pamil/Sylius,kayue/Sylius,mbabker/Sylius,Zales0123/Sylius,Sylius/Sylius,101medialab/Sylius,pamil/Sylius,loic425/Sylius,antonioperic/Sylius,kayue/Sylius,foobarflies/Sylius