Bump io.github.problem4j:problem4j-core from 1.3.0 to 1.3.1 (#24)

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gradle/libs.versions.toml

@@ -3,7 +3,7 @@ jackson2 = "2.20.1"
 jackson3 = "3.0.3"
 junit = "6.0.2"
 nmcp = "1.4.3"
-problem4j-core = "1.3.0"
+problem4j-core = "1.3.1"
 spotless = "8.1.0"
 
 [plugins]