Maven artifact signing on CI for releases

Needs

```groovy
plugins.withId('com.vanniktech.maven.publish') {
signing {
def signingKey = findProperty('signingKey')
def signingPassword = ''
useInMemoryPgpKeys(signingKey, signingPassword)
}
}
```

And a GPG key without a password added to secrets.

Locate the release CI configuration and the Gradle publishing setup that applies com.vanniktech.maven.publish. Check how the signingKey secret and GPG key should reach the release job, then verify that a release produces signed Maven artifacts.

Written by the indexing model from the issue text.