Fix FafAuthenticationConverter

[bukajsytlos]

faf-java-api  | java.lang.ClassCastException: class com.nimbusds.jose.shaded.gson.internal.LinkedTreeMap cannot be cast to class com.nimbusds.jose.shaded.gson.JsonObject (com.nimbusds.jose.shaded.gson.internal.LinkedTreeMap and com.nimbusds.jose.shaded.gson.JsonObject are in unnamed module of loader org.springframework.boot.loader.LaunchedURLClassLoader @610455d6)                                                                  faf-java-api  |         at com.faforever.api.security.FafAuthenticationConverter.extractUsername(FafAuthenticationConverter.java:34)
faf-java-api  |         at com.faforever.api.security.FafAuthenticationConverter.convert(FafAuthenticationConverter.java:22)