minor symfony#18151 Improved the "branch" row of the PR table (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

Improved the "branch" row of the PR table

| Q             | A
| ------------- | ---
| Branch        | 2.3+
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#18128
| License       | MIT
| Doc PR        | -

Commits
-------

6af23cc Improved the "branch" row of the PR table

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q             | A
 | ------------- | ---
-| Branch        | master for features and deprecations / lowest applicable and maintained version otherwise
+| Branch?       | "master" for new features / 2.3, 2.7, 2.8 or 3.0 for fixes
 | Bug fix?      | yes/no
 | New feature?  | yes/no
 | BC breaks?    | yes/no