[Maintenance] Update PR template (Sylius#395)

| Q               | A
| --------------- | -----
| Branch?         | 2.1
| Bug fix?        | no
| New feature?    | no
| Related tickets | replaces Sylius#393

Author: GSadee

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q               | A
 | --------------- | -----
-| Branch?         | 1.5 (security fixes), 1.6 (bug fixes, improvements), 1.7 (new features)
+| Branch?         | 1.6 (security fixes), 1.7 and 2.0 (bug fixes, improvements), 2.1 (new features)
 | Bug fix?        | no/yes
 | New feature?    | no/yes
 | Related tickets | fixes #X, partially #Y, mentioned in #Z