Upgrade apache-rat-plugin to version 0.15

Fixed Bugs:
o RAT-309: Site builds could not be generated properly due to API
changes: solution was to upgrade Maven Reporting API to 3.1.1 and use
details of Maven Reporting Implementation 3.2.0 in RAT's Mojo hierarchy.
Thanks to Michael Osipov, Gary Gregory.
o RAT-309: Updated internal data structures from deprecated ArrayStack
to JDK's ArrayDeque.
o RAT-306: Add note about hierarchy of changelogs in RAT project
structure when publishing the project webpage.
o RAT-307: Update to focal (Ubuntu 20.04) on Travis to circumvent build
errors and be able to use more modern JDK versions. Deprecate openJDK8
build with focal as it is not supported on Travis.

Changes:
o RAT-305: Update maven-project-info-reports-plugin from 3.3.0 to
3.4.1. Thanks to dependabot.
o RAT-305: Update maven-javadoc-plugin from 3.4.0 to 3.4.1. Thanks to
dependabot.
o RAT-305: Update maven-jxr-plugin from 3.2.0 to 3.3.0. Thanks to
dependabot.
o RAT-305: Update animal-sniffer-maven-plugin from 1.21 to 1.22. Thanks
to dependabot.
o RAT-305: Update maven-site-plugin from 3.12.0 to 3.12.1. Thanks to
dependabot.
o RAT-305: Update maven-pmd-plugin from 3.16.0 to 3.18.0. Thanks to
dependabot.
o RAT-305: Update mockito-core from 4.6.0 to 4.7.0. Thanks to dependabot.
o RAT-305: Update extra-enforcer-rules from 1.5.1 to 1.6.1. Thanks to
dependabot.
o RAT-305: Update Apache parent pom from 26 to 27. Thanks to dependabot.
o RAT-305: Update wagon-ssh from 3.5.1 to 3.5.2. Thanks to dependabot.
o RAT-305: Update maven-enforcer-plugin from 3.0.0 to 3.1.0. Thanks to
dependabot.
o RAT-305: Update actions/setup-java from 3.3.0 to 3.4.1. Thanks to
dependabot.
o RAT-310: Fix deprecation warnings in tests. Use hamcrest's
annotations instead of ones from JUnit.

https://creadur.apache.org/rat/