remove support for Ubuntu 18.04.

[ernstleierzopf]

Make sure these boxes are signed before submitting your Pull Request -- thank you.

Must haves

• I have read and followed the contributing guide lines at https://github.com/ait-aecid/logdata-anomaly-miner/wiki/Git-development-workflow
• Issues exist (in ttps://github.com/ait-aecid/logdata-anomaly-miner) for this PR
• I added related issues using the "Fixes #"-notations
• This Pull-Requests merges into the "development"-branch

Fixes #

Submission specific

• This PR adds required modules to the aminer installation
• This PR changes installations of existing modules
• This PR changes the default python version for the aminer

Describe changes: