minor symfony#18942 Add 3.1 to PR template branch row and remove 2.3 (chalasr)

This PR was merged into the 2.7 branch.

Discussion
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Add 3.1 to PR template branch row and remove 2.3

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
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71dcaa1 Add 3.1 to  PR template branch row, remove 2.3

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q             | A
 | ------------- | ---
-| Branch?       | "master" for new features / 2.3, 2.7, 2.8 or 3.0 for fixes
+| Branch?       | "master" for new features / 2.7, 2.8, 3.0 or 3.1 for fixes
 | Bug fix?      | yes/no
 | New feature?  | yes/no
 | BC breaks?    | yes/no