Avoid UnsupportedEncodingException by design, replacing magic string "UTF-8" with StandardCharsets.UTF_8 #624

Job	Run time
Build with JDK17 (samples/openapi3/client/petstore/spring-cloud-3)	1m 21s
Build with JDK17 (samples/client/petstore/java-helidon-client/v3/mp/)	44s
Build with JDK17 (samples/client/petstore/java-helidon-client/v3/se)	55s
Build with JDK17 (samples/client/petstore/spring-http-interface-reactive)	1m 28s
Build with JDK17 (samples/client/petstore/spring-http-interface)	33s
Build with JDK17 (samples/client/petstore/java/webclient-jakarta)	42s
Build with JDK17 (samples/openapi3/server/petstore/springboot-3)	1m 7s
Build with JDK17 (samples/server/petstore/java-helidon-server/v3/mp/)	1m 7s
Build with JDK17 (samples/server/petstore/java-helidon-server/v3/se)	59s
Build with JDK17 (samples/client/petstore/spring-http-interface-reactive)	42s
Build with JDK17 (samples/client/petstore/spring-http-interface)	34s
	10m 12s

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