minor #14352 Replace master by 5.x (fabpot)

This PR was merged into the 3.4 branch.

Discussion
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Replace master by 5.x

<!--

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
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Commits
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16470d8 Replace master by 5.x

Author: OskarStark

.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/roadmap 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 `master` for features of unreleased versions).
+the feature was introduced (and `5.x` for features of unreleased versions).
 
 -->