[INTERNAL] Move test infrastructure from azure to github actions

[flovogt]

• Migrating test matrix execution from azure to github actions
• Remove azure / azure coverage specific coding and dependencies
• Update badge url in README.md
• use node v20 instead of outdated 19

[coveralls]

coverage: 94.589%. remained the same when pulling 8069ac6 on gh-migration into 41fa2d6 on main.

[RandomByte]

As discussed, since we fixed the Azure pipeline with #923 and #921, we stick to Azure for the time being.