Draft Release 4: Basic Changes

[KarstenSchnitter]

This is the first PR on the way to #164. It contains the following major changes:

• deletion of custom metrics feature
• removal of Jersey support and old sample app
• remove javax.servlet support in favour of Jakarta
• migrate to Java 17 (at Java 11 code level)
• upgrade to SLF4J 2.x and Logback/Log4j2 to latest versions
• upgrade JUnit and Mockito to v5
• replacement of Hamcrest by AssertJ