Swap `/pom.xml` and `/log4j-bom/pom.xml`

- `/pom.xml` is moved to `/log4j-parent/pom.xml`
- `/log4j-bom/pom.xml` is moved to `/pom.xml`
- Implements the BOM organization described by Maven[1].
  That is, `parent` inherits from `bom`.
- Takes advantage of `flatten-bom` provided by `logging-parent`
- Identical scheme to the one found in `-tools` and `-transformation`

[1] https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#bill-of-materials-bom-poms