minor symfony#36942 Update pull request template for 5.1 (derrabus)

This PR was merged into the 3.4 branch.

Discussion
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Update pull request template for 5.1

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

Commits
-------

d4b598c Update pull request template for 5.1.

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q             | A
 | ------------- | ---
-| Branch?       | master for features / 3.4, 4.4 or 5.0 for bug fixes <!-- see below -->
+| Branch?       | master for features / 3.4, 4.4, 5.0 or 5.1 for bug fixes <!-- see below -->
 | Bug fix?      | yes/no
 | New feature?  | yes/no <!-- please update src/**/CHANGELOG.md files -->
 | Deprecations? | yes/no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->