Bump com.diffplug.spotless from 8.4.0 to 8.6.0 (#107)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -9,7 +9,7 @@ nullaway = "0.13.4"
 problem4j-core = "2.0.0"
 problem4j-jackson2 = "2.0.0"
 problem4j-jackson3 = "2.0.0"
-spotless = "8.4.0"
+spotless = "8.6.0"
 spring-boot = "4.0.6"
 
 [plugins]