JS Bin environment in Contributing page still uses Bootstrap v4
JS Bin environment mentioned in CONTRIBUTING.md still uses v4. We should update it to use v5.
The other way around would be to modify CONTRIBUTING.md to suggest the same think as in the bug_report.yml: suggested fixes using CodePen ([v4 template](https://codepen.io/team/bootstrap/pen/yLabNQL) or [v5 template](https://codepen.io/team/bootstrap/pen/qBamdLj)).
Thanks @Lausselloic for having reported this issue