Bump highlight.js version to 9.15.8 release
Bump highlight.js version to 9.15.8 release

Latest highlight.js releases introduced some improvements: [1].

[1] https://github.com/highlightjs/highlight.js/blob/9.15.8/CHANGES.md

Change-Id: I97dc6a0c17a2c659aa8777afa8d6d5c0bd95ba2b

GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit