chore: migrate to Junit 5

[felix-seifert]

The main purpose of this PR is to migrate to Junit 5. Even though we already used some Junit 5 tests, not all tests were migrated. This led to cases where not all tests could be discovered.

With the migration, I reduced the number of direct dependencies and abolished the need to depend on a few old dependencies. Namely, I updated Junit to use the Jupiter Engine and the Jupiter API, okhttp, Mockito, objenesis and the Surefire plugin. Furthermore, I removed the outdated Powermock dependencies because the mapping of final classes is now possible in Mockito.

Side note: Even though the used okhttp packages (mockwebserver and okhttp) could be updated to version 4.12.0, I discovered that they indirectly depend on different versions of kotlin-stdlib-jdk8 and just used version 4.11.0.

[codecov]

Codecov Report

Merging #157 (4bd19c3) into master (4fa26d8) will increase coverage by 0.60%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #157      +/-   ##
============================================
+ Coverage     75.83%   76.44%   +0.60%     
- Complexity      284      286       +2     
============================================
  Files            42       42              
  Lines           985      985              
  Branches         43       43              
============================================
+ Hits            747      753       +6     
+ Misses          212      207       -5     
+ Partials         26       25       -1     

see 1 file with indirect coverage changes

[dennisgranath]

LGTM