Upgrade to Prometheus Client 1.3.8

Closes gh-45837

spring-boot-project/spring-boot-dependencies/build.gradle

@@ -1797,11 +1797,7 @@ bom {
 			releaseNotes("https://github.com/pgjdbc/pgjdbc/releases/tag/REL{version}")
 		}
 	}
-	library("Prometheus Client", "1.3.6") {
-		prohibit {
-			versionRange "[1.3.7]"
-			because "its bom has dependency management for JUnit (https://github.com/prometheus/client_java/issues/1352)"
-		}
+	library("Prometheus Client", "1.3.8") {
 		group("io.prometheus") {
 			bom("prometheus-metrics-bom")
 		}