Use Junit5 as base test framework

[bitxon]

Description

• Use Junit5 as default test framework
• Keep few Junit4 tests as an examples
• Update README.md according to latest changes

References

N/A

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• The repository's code style is followed (see the contributing guide)
• Test coverage that demonstrates that the change works as expected
• For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org