@nuxtjs/i18n: breaking changes for update, update package

Current status: Until now we used "@nuxtjs/i18n": "^8.0.0-beta.10". We need to use a beta because nuxt3 is supported from this package only from 8.0.0 on. Now there was a new beta release but with beta.11 there are breaking changes and starting will fail:

Cannot start nuxt:  [@nuxtjs/i18n]: The vueI18n option is no longer be specified with object.                                                                                                                        
It must be specified in the configuration file via the 'i18n.config' path.
About deprecated reason, see https://v8.i18n.nuxtjs.org/guide/migrating#change-the-route-key-rules-in-pages-option
About new configuration style, sqee https://v8.i18n.nuxtjs.org/getting-started/basic-usage#translate-with-vue-i18n

Good thing: The solution described in the link (creating i18n.config.ts and move i18n options from nuxt.config.ts over there) works well. Bad thing: It seems that handling message keys in files isn't implemented for now (or at leas I didn't find it…). But then the content of our language files is not found anymore. Need either wait for a new (final) version or do some deeper investigation on that.

For now I will fix 8.0.0-beta.10 as a version.