Try to improve workflows using new actions

[mathze]

Context

Current workflows are quite old. Since their creation many new actions have emerged, which might improve our workflows.

Goal

We want to

• try to reduce the overall footprint of the workflows
  • maintenance -> no. of actions, action-configuration
  • performance -> reduce execution time, optimize cache-entries & size
    clean cache entries only used within a workflow run (e.g. those for code coverage)
• re-evaluate existing actions & configurations

Hints

• Evaluate gradle/gradle-build-action, alternative to burrunan/gradle-cache-action?
• Report upload & generation (does codecov need aggregate?)
• actions/cache vs. dedicated actions/cache/restore and actions/cache/save