Update actions/checkout action to v7 (#953) #92

Job	Run time
Build scip-java CLI	4m 17s
maven-kotlin-json-kotlin-maven	2m 34s
gradle-kotlin-flowext	4m 4s
maven-java25-demo	1m 33s
gradle-java21-readiness	2m 34s
maven-java17-spring-boot	2m 43s
gradle-kotlin-okio	4m 55s
gradle-mixed-okio-jmh	4m 11s
bazel-java-examples	2m 44s
	29m 35s

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