Bump vue-i18n from 8.20.0 to 8.21.0 in /frontend

Bumps [vue-i18n](https://github.com/kazupon/vue-i18n) from 8.20.0 to 8.21.0.
- [Release notes](https://github.com/kazupon/vue-i18n/releases)
- [Changelog](https://github.com/kazupon/vue-i18n/blob/v8.x/CHANGELOG.md)
- [Commits](https://github.com/kazupon/vue-i18n/compare/v8.20.0...v8.21.0)

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

frontend/package.json

@@ -19,7 +19,7 @@
         "sass": "^1.26.10",
         "sass-loader": "^9.0.3",
         "vue": "^2.6",
-        "vue-i18n": "^8.20",
+        "vue-i18n": "^8.21",
         "vue-template-compiler": "^2.6.11"
     },
     "dependencies": {