...otherwise it won't play nice with the bootstrap context

I removed the unecessary plugins from the build and reworked it to use
the ci friendly way of setting the version for releases (see
https://maven.apache.org/maven-ci-friendly.html).

This removes the spring-boot-starter-parent from the build, and
duplicates the necessary plugin configuration in he Spring Boot Admin'
parent instead. This way our BOM doesn't include version for
dependencies managed by boot's BOM