Upgrade to serenity-core version without runtime dependency on JUnit4

[fabianlinz]

From the readme...

• serenity-core and serenity-model have dependencies to JUnit4, which has several implications.
  • At runtime JUnit4 is currently still needed.
  • serenity-core and serenity-model should be changed such that JUnit4 is an optional dependency.
• Automatically taking screenshot for web tests
  • Screenshots are only taken if TestCaseAnnotations.isASerenityTestCase is true. Currently this method only works for JUnit4 tests as it checks for the annotation @RunWith
  • Workaround: Tests can extend net.serenitybdd.junit5.AbstractSerenityTestDetectionWorkaround

[fabianlinz]

Issue in serenity-core: https://github.com/serenity-bdd/serenity-core/issues/1858

[amohrig]

Hi @fabianlinz ,

I created branch upgrade_serenity_core and pushed a first attempt.