solr 7.5 config does not work with search_api_solr module v3.x

[fmuellerprevion]

Describe the bug
When using the newest version(s) of the search_api_solr module, a correct setup with solr 7.5 cannot be used when using the standard config provided in this repository.

To Reproduce
Steps to reproduce the behavior:

1. Set up D8 with the module search_api and search_api_solr v3.x
2. Set up a solr container which builds it using the amazeeio/solr:7.5 image and having the config from https://github.com/amazeeio/lagoon/tree/master/images/solr-drupal/solr7.5/conf
3. Set up the solr server and an index in the config
4. See error

Expected behavior
On the solr server page under search API, there should be no error and the server should be reachable. Views on the created index should deliver a correct result.

Actual behavior
The server page reads the message:

You are using outdated Solr configuration files. Please follow the instructions in the INSTALL.md file for setting up Solr.

When running a query using a view, this error is thrown:

Error while checking server index status: An error occurred while trying to search with Solr: { "error":{ "metadata":[ "error-class","org.apache.solr.common.SolrException", "root-error-class","org.apache.solr.common.SolrException"], "msg":"can not use FieldCache on multivalued field: boost_document", "code":400}}

A support request on the module is here, but which seems to be a configuration issue: https://www.drupal.org/project/search_api_solr/issues/3056971

Additional context
Downgrading the search_api_solr module version fro 3.x to 2.7 works for Solr 7.5. However I had to remove the volume and build again to get it to work correctly.

[vincenzodnp]

Hello @fmuellerprevion are you not able to index content at all?
I was able but still get the error in the admin page.
Thanks
Vincenzo

[vincenzodnp]

@fmuellerprevion we are going to update the solr config of our solr:7.5-drupal image
Thanks for your support
Vincenzo