Bump gulp-terser from 1.4.1 to 2.1.0

Bumps [gulp-terser](https://github.com/duan602728596/gulp-terser) from 1.4.1 to 2.1.0.
- [Release notes](https://github.com/duan602728596/gulp-terser/releases)
- [Commits](duan602728596/gulp-terser@v1.4.1...v2.1.0)

---
updated-dependencies:
- dependency-name: gulp-terser
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -39,7 +39,7 @@
   "dependencies": {
     "@fancyapps/fancybox": "^3.5.7",
     "gulp-concat": "^2.6.1",
-    "gulp-terser": "^1.4.0",
+    "gulp-terser": "^2.1.0",
     "jquery": "^3.5.1",
     "techy": "^1.2.5"
   }