Fix Python issues and improve variable naming and their defaults

Lock Python version to ensure consistent results

• use Python 3.9;
• use Certifi package to fix the SSL/TLS issues.

Rename variables GROUP_ID to GITLAB_GROUPS

• value is supposed to be name, not ID;
• GROUPS would interfere with the OS groups variable.

Introduce better defaults

• the GITLAB_URL defaults to the predefined CI_SERVER_URL var which means no need to tweak this value at all;
• default to the current group/namespace to make it work on both Self-Hosted and SaaS versions of GitLab;
• by default, only GITLAB_GROUPS groups are reported. However, if the GITLAB_INSTANCE_WIDE is set to non-zero value (defaults to 0), instance-wide report will be generated.