I see this error moving from java 8 to 11 in AEM 6.5. Execution generate-scr-scrdescriptor of goal org.apache.felix:maven-scr-plugin:1.20.0:scr failed: org.apache.felix.scrplugin.annotations.AnnotationProcessor is not an ImageIO SPI class?
The other option will be to start using the maven-bundle-plugin and the standard Declarative Services annotations instead of the Custom Felix annotations. Note that this would still involve updating those plugins to versions which can support Java 11. I personally using maven-bundle-plugin version 4.2.0 with Java 11 and it works perfectly fine.