Update minimum Python version to 3.8

[tomuben]

Background

Currently the minimum version is set to Python 3.6. However, Python 3.6 is already EOL, and many packages stopped support for it.

• Requires
  • Remove support for python version < 3.8 exasol-python-test-framework#37
  • Update minimum Python version to 3.8 integration-test-docker-environment#192

Acceptance Criteria

• Update minimum Python version to 3.8 in Poetry
• Update Docker-Start-Container to Ubuntu 20.04 and Python3.8 (https://github.com/exasol/script-languages-container-tool/blob/main/docker_runner/Dockerfile)
• Update Docker-Test-Container to Ubuntu 20.04 and Python 3.8 (https://github.com/exasol/script-languages-container-tool/blob/main/tests/Dockerfile)