Outdated Bouncy Castle

The version of Bouncy Castle currently in [use](https://mvnrepository.com/artifact/org.bouncycastle/bcprov-jdk15on/1.70) is outdated. It not only has two known vulnerabilities, but it is also incompatible with the latest [digidoc4j library](https://github.com/open-eid/digidoc4j/releases).
