Bump org.pitest:pitest-maven from 1.15.3 to 1.15.6 (#835)

Bumps [org.pitest:pitest-maven](https://github.com/hcoles/pitest) from 1.15.3 to 1.15.6.
- [Release notes](https://github.com/hcoles/pitest/releases)
- [Commits](hcoles/pitest@1.15.3...1.15.6)

---
updated-dependencies:
- dependency-name: org.pitest:pitest-maven
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>