Cleanup libquassel to also run under JDK6

build.gradle.kts

...de/justjanne/coverageconverter/CoverageConverterAction.kt

gradle.properties

...lin/de/justjanne/libquassel/protocol/io/ByteBufferUtil.kt

.../de/justjanne/libquassel/protocol/io/ChainedByteBuffer.kt

...tlin/de/justjanne/libquassel/protocol/io/StringEncoder.kt

...in/kotlin/de/justjanne/libquassel/protocol/util/Buffer.kt

...ibquassel/protocol/serializers/qt/StringSerializerTest.kt

...e/libquassel/protocol/testutil/handshakeSerializerTest.kt

...ibquassel/protocol/testutil/matchers/ByteBufferMatcher.kt

...e/libquassel/protocol/testutil/primitiveSerializerTest.kt

.../justjanne/libquassel/protocol/testutil/serializerTest.kt

...libquassel/protocol/testutil/signalProxySerializerTest.kt