SpringBoot Change references to Spring Cloud Sleuth for SpanContextSupplier to Micrometer Tracing

https://github.com/spring-projects/spring-boot/blob/main/spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/exemplars/ExemplarsAutoConfiguration.java

SpanContextSupplier is autoconfigured if a Tracer from Micrometer Tracing is present, not Spring Cloud Sleuth

Comment From: wilkinsona

Thanks very much, @making.