Update migration tool's highlight.js to one that supports Null Safety keywords

[srawlins]

There isn't a release yet, but grab a release which includes highlightjs/highlight.js#2551 (v10.1.0 perhaps)

[srawlins]

highlight.js 10.1.0 has Null Safety keywords, and nullable built-in types (int? etc). https://highlightjs.org/