minor symfony#52601 Update Github template for 7.1 (fabpot)

This PR was merged into the 5.4 branch.

Discussion
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Update Github template for 7.1

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        | n/a
| License       | MIT

n/a

Commits
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c85d118 Update Github template for 7.1

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q             | A
 | ------------- | ---
-| Branch?       | 6.4 for features / 5.4 or 6.3 for bug fixes <!-- see below -->
+| Branch?       | 7.1 for features / 5.4, 6.3, 6.4, or 7.0 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 -->