Cater for version constraints in _TARGET_CORE value
This is a follow-up to discussions on Slack https://drupal.slack.com/archives/CGKLP028K/p1689677147300149
The variable _TARGET_CORE currently only supports specifying fixed two-level and three-level core versions, eg 9.5 and 9.5.4. For a project branch which is not compatible with D10 this variable has to be overridden in the .gitlab-ci.yml, which will require a new commit to the when Core 9.6 is released, then 9.7, etc.
It would be nice to use constraint syntax such as ^9.2 or ~9. These do load the required version of Core but fail on version_compare(getenv('_TARGET_CORE'), '9.0.0', '>='))