Upgrade Spring Framework to 5.3.15

Closes gh-1998

Author: eleftherias

gradle/dependency-management.gradle

@@ -3,7 +3,7 @@ dependencyManagement {
 		mavenBom 'io.projectreactor:reactor-bom:2020.0.15'
 		mavenBom 'com.fasterxml.jackson:jackson-bom:2.13.1'
 		mavenBom 'org.junit:junit-bom:5.8.2'
-		mavenBom 'org.springframework:spring-framework-bom:5.3.14'
+		mavenBom 'org.springframework:spring-framework-bom:5.3.15'
 		mavenBom 'org.springframework.data:spring-data-bom:2021.1.0'
 		mavenBom 'org.springframework.security:spring-security-bom:5.6.1'
 		mavenBom 'org.testcontainers:testcontainers-bom:1.16.2'