relicense shootout-regex-dna.rs

Part of #14248

The authors are @pcwalton and @BurntSushi, and we have their agreement.
auto merge of #14728 : TeXitoi/rust/relicense-shootout-regex-dna, r=brson

Part of #14248

The authors are @pcwalton and @BurntSushi, and we have their agreement.

@brson OK?