Include netty-handler as transient dependency of cassandra-driver-dse

Previously, Spring Data Cassandra excluded Netty from the Cassandra
Driver's dependencies. This was an error that we replicated in
our dependency management. The exclusion has been removed from
Spring Data Cassandra's pom. This commit makes the equivalent update
to our dependency management.

Fixes spring-projectsgh-6616
Closes spring-projectsgh-6721

spring-boot-dependencies/pom.xml

@@ -596,10 +596,6 @@
 						<groupId>com.google.guava</groupId>
 						<artifactId>guava</artifactId>
 					</exclusion>
-					<exclusion>
-						<groupId>io.netty</groupId>
-						<artifactId>netty-handler</artifactId>
-					</exclusion>
 				</exclusions>
 			</dependency>
 			<dependency>