minor symfony#59212 Remove 5.4 branch from PR template (chalasr)

This PR was merged into the 6.4 branch.

Discussion
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Remove 5.4 branch from PR template

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

Not maintained anymore

Commits
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0332f5e Remove 5.4 branch from PR template

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q             | A
 | ------------- | ---
-| Branch?       | 7.3 for features / 5.4, 6.4, 7.1, and 7.2 for bug fixes <!-- see below -->
+| Branch?       | 7.3 for features / 6.4, 7.1, and 7.2 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 -->