Ask to open PRs for new features in 6.x

Author: dunglas

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,6 @@ If your pull request fixes a BUG, use the oldest maintained branch that contains
 the bug (see https://symfony.com/releases for the list of maintained branches).
 
 If your pull request documents a NEW FEATURE, use the same Symfony branch where
-the feature was introduced (and `5.x` for features of unreleased versions).
+the feature was introduced (and `6.x` for features of unreleased versions).
 
 -->