Add dependency management for org.apache.httpcomponents:httpcore

Without dependency management for org.apache.httpcomponents:httpcore,
it’s possible to get mismatched versions of httpcore and httpclient.

Closes gh-2941

spring-boot-dependencies/pom.xml

@@ -822,6 +822,11 @@
 					</exclusion>
 				</exclusions>
 			</dependency>
+			<dependency>
+				<groupId>org.apache.httpcomponents</groupId>
+				<artifactId>httpcore</artifactId>
+				<version>${httpclient.version}</version>
+			</dependency>
 			<dependency>
 				<groupId>org.apache.httpcomponents</groupId>
 				<artifactId>httpmime</artifactId>