build(sample): bumped spring-boot to 2.3.1, kotlin to 1.3.72 and ff4j to 1.8.7
build(sample): bumped spring-boot to 2.3.1, kotlin to 1.3.72 and ff4j to 1.8.7

paul58914080/ff4j-spring-boot-starter-parent,paul58914080/ff4j-spring-boot-starter-parent