Bump gulp-nunjucks from 5.1.0 to 6.0.0

Bumps [gulp-nunjucks](https://github.com/sindresorhus/gulp-nunjucks) from 5.1.0 to 6.0.0.
- [Release notes](https://github.com/sindresorhus/gulp-nunjucks/releases)
- [Commits](sindresorhus/gulp-nunjucks@v5.1.0...v6.0.0)

---
updated-dependencies:
- dependency-name: gulp-nunjucks
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -45,7 +45,7 @@
     "gulp-clean": "^0.4.0",
     "gulp-clean-css": "^4.3.0",
     "gulp-connect": "^5.7.0",
-    "gulp-nunjucks": "^5.1.0",
+    "gulp-nunjucks": "^6.0.0",
     "gulp-rename": "^2.0.0",
     "gulp-sass": "^6.0.0",
     "gulp-uglify": "^3.0.2",