Bump spring-boot-dependencies from 1.5.15.RELEASE to 1.5.21.RELEASE#43

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dependabot/maven/org.springframework.boot-spring-boot-dependencies-1.5.21.RELEASEzalando-stups/kio-client-java:dependabot/maven/org.springframework.boot-spring-boot-dependencies-1.5.21.RELEASECopy head branch name to clipboard