Add dependency management for logback-core

Previously, only dependency management for logback-classic was
provided. This meant that it was possible for logback-core, upon
which logback-classic depends, to have a different version.

This commit adds dependency management for logback-core, thereby
ensuring that the two dependencies will have the same version.

Closes gh-5304

spring-boot-dependencies/pom.xml

@@ -485,6 +485,11 @@
 				<artifactId>logback-classic</artifactId>
 				<version>${logback.version}</version>
 			</dependency>
+			<dependency>
+				<groupId>ch.qos.logback</groupId>
+				<artifactId>logback-core</artifactId>
+				<version>${logback.version}</version>
+			</dependency>
 			<dependency>
 				<groupId>com.atomikos</groupId>
 				<artifactId>transactions-jdbc</artifactId>
@@ -2235,4 +2240,4 @@
 			<id>integration-test</id>
 		</profile>
 	</profiles>
-</project>
+</project>
\ No newline at end of file