Update to config split 2.x for patching capability

Problem/Motivation

Currently the project depends on config_split 1.x, with a "develop" split configured.
The Belgian (and other) websites are using config split in order to split of language-specific configuration (e.g. enable FR language for the .be website, which should not happen in the general project).

I propose to update the config_split module to its version 2.x as soon as possible, because that introduces the concept of patching. Rather than splitting a whole config YML file, config_split only puts the changes in the split directory.

Steps to reproduce

The default language.negotiation.yml looks like this:

session:
  parameter: language
url:
  source: path_prefix
  prefixes:
    en: en
    '': null
  domains:
    en: ''
selected_langcode: site_default

When using config_split 1.x, we split the whole file. If later the "session" or "selected_langcode" property would change in the default config, it will not propagate to the split off configurations.

Proposed resolution

Use config_split 2.x, so the above configuration would be stored in config/drupal_be split folder as follows:

adding:
  url:
    prefixes:
      nl: nl
      fr: fr
      de: de
    domains:
      nl: ''
      fr: ''
      de: ''
removing:
  url:
    prefixes:
      '': null

This alters only the prefixes content, nothing else.

If we decide to let all spin-off sites use config_split, it will be a lot more maintainable this way.

Remaining tasks

- Create MR to update to config_split 2.x + update existing splits