Update gulp-sourcemaps requirement to 2.6.4 in /blueocean-core-js#23

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dependabot/npm_and_yarn/blueocean-core-js/gulp-sourcemaps-2.6.4michaelneale/blueocean-plugin:dependabot/npm_and_yarn/blueocean-core-js/gulp-sourcemaps-2.6.4Copy head branch name to clipboard