Solarium 5.0.2 is too old and has known issues. In order to get composer to upgrade solarium to a recent version a core patch or a workaround is required. Have a look at this Core issue and this Search API Solr issue.

composer require symfony/event-dispatcher:"4.3.4 as 3.4.99"

 

Do not run Composer as root/super user! See https://getcomposer.org/root for details
./composer.json has been updated
Gathering patches for root package.
No patches supplied.
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 1 update, 0 removals
Gathering patches for root package.
No patches supplied.
Gathering patches for dependencies. This might take a minute.
  - Installing symfony/event-dispatcher-contracts (v1.1.7): Downloading (100%)
  - Updating symfony/event-dispatcher (v3.4.36 => v4.3.4): Downloading (100%)
symfony/event-dispatcher-contracts suggests installing psr/event-dispatcher
Writing lock file
Generating autoload files

composer update

 

Do not run Composer as root/super user! See https://getcomposer.org/root for details
Gathering patches for root package.
No patches supplied.
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 1 update, 5 removals
  - Removing symfony/var-exporter (v4.4.2)
  - Removing symfony/service-contracts (v1.1.8)
  - Removing symfony/cache-contracts (v1.1.7)
  - Removing symfony/cache (v4.3.9)
  - Removing psr/cache (1.0.1)
Gathering patches for root package.
No patches supplied.
Gathering patches for dependencies. This might take a minute.
  - Updating solarium/solarium (5.0.3 => 5.1.5): Downloading (100%)
Writing lock file
Generating autoload files

SolrCore Initialization Failures
site5: org.apache.solr.common.SolrException:org.apache.solr.common.SolrException: Could not load conf for core itlntk: Can't load schema /var/solr/data/site5/conf/schema.xml: Plugin init failure for [schema.xml] fieldType "collated_en": Error loading class 'solr.ICUCollationField'
Please check your logs for more information