Update Gradle wrapper to version 8.13

[SimonScholz]

With this update also Java 21 can be used to built this project.

I simply ran gradle wrapper --gradle-version 8.13 on my machine ;)

[gmuth]

It looks like no code or build files changes are required to build the library. So whoever wants to use gradle 8 can do this.

However according to the official documentation this combination of build tools and their versions is not supported. The build process also includes tests, generating javadoc, running sonar scans and publishing artifacts. Upgrading usually means leaving older runtimes behind with no support. The last upgrade (Gradle 7, Kotlin 1.7 away from Java 1.6) was a breaking change for some android devices. For this or future upgrades it would be great to have compatibility checks automated e.g. in github actions or gradle tasks. For compatibility reasons I want to stick with Gradle 7, Kotlin 1.7 and Java 8 support.