Bump bootstrap-select from 1.6.3 to 1.13.6 in /shuup/themes/classic_gray

Bumps [bootstrap-select](https://github.com/snapappointments/bootstrap-select) from 1.6.3 to 1.13.6.
- [Release notes](https://github.com/snapappointments/bootstrap-select/releases)
- [Changelog](https://github.com/snapappointments/bootstrap-select/blob/v1.13.0-dev/CHANGELOG.md)
- [Commits](snapappointments/bootstrap-select@v1.6.3...v1.13.6)

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

shuup/themes/classic_gray/package.json

@@ -16,7 +16,7 @@
     "autoprefixer": "^9.6.1",
     "babel-preset-env": "^1.7.0",
     "bootstrap": "^3.4.1",
-    "bootstrap-select": "1.6.3",
+    "bootstrap-select": "1.13.6",
     "dropzone": "^5.5.1",
     "font-awesome": "^4.7.0",
     "jquery": "^3.4.1",