Remove User-Agent from vary header to improve cache-hit-ratio on CDN

### Expected Behaviour

CDN or proxy is caching resources with high cache hit ratio.

### Actual Behaviour

This is not the case as AEM per default adds the user agent to the vary header, that forces the CDN/proxies to add the user agent string to the cache-key, hence making caching ineffective. This is most likely a legacy setting, back from when CMS would render different output based on user agent string - e.g using WURFL for device detection and adjusting the output. AEM is no longer doing that and it's not recommended to do this anymore.

For example see:

See line 43 in:
https://github.com/adobe/aem-project-archetype/blob/master/src/main/archetype/dispatcher.cloud/src/conf.d/available_vhosts/default.vhost

See line 20 in
https://github.com/adobe/aem-project-archetype/blob/master/src/main/archetype/dispatcher.ams/src/conf.d/available_vhosts/aem_publish.vhost

More:
https://github.com/adobe/aem-project-archetype/search?q=Header+append+Vary+User-Agent+env%3D%21dont-vary

This has already been fixed in the reference https://git.corp.adobe.com/Granite/dispatcher-k8s-base/pull/48/files and now also needs to be fix in the project archetype. wknd.site , aemcomponents.dev and Odin have been updated with the updated config.

### Reproduce Scenario (including but not limited to)

#### Steps to Reproduce

#### Platform and Version

#### Sample Code that illustrates the problem

#### Logs taken while reproducing problem

Start with the two named vhost files: src/main/archetype/dispatcher.cloud/src/conf.d/available_vhosts/default.vhost and src/main/archetype/dispatcher.ams/src/conf.d/available_vhosts/aem_publish.vhost, then compare their Vary handling with the referenced dispatcher-k8s-base change. Done means the generated archetype configuration no longer adds User-Agent to Vary and the affected configuration remains valid.

Written by the indexing model from the issue text.