relicense shootout-mandelbrot.rs

Part of #14248

Main authors:
- @Ryman: OK
- @TeXitoi: OK
- @pcwalton: OK

Minor authors:
- @brson: OK
- @alexcrichton: OK
- @kballard: OK

Remark: @tedhorst was a main contributor, but its contribution
disapear with @pcwalton rewrite at af4ea11

@brson OK?