JS Bin environment in Contributing page still uses Bootstrap v4

[julien-deramond]

JS Bin environment mentioned in CONTRIBUTING.md still uses v4. We should update it to use v5.

bootstrap/.github/CONTRIBUTING.md

Line 72 in 6f10bdc

[This JS Bin](https://jsbin.com/lolome/edit?html,output) is a helpful template.

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