Switch validator starter for hibernate validator

In the web starter we shouldn't depend explicitly on any EL
implementation, otherwise when people build wars and deploy
them in containers that have their own EL there is a conflict.
We can still depend on hibernate-validator to support
JSR-303 in web apps because the EL implementation comes with
the container.

Fixes gh-5454

spring-boot-starters/spring-boot-starter-web/pom.xml

@@ -28,8 +28,8 @@
 			<artifactId>spring-boot-starter-tomcat</artifactId>
 		</dependency>
 		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-validation</artifactId>
+			<groupId>org.hibernate</groupId>
+			<artifactId>hibernate-validator</artifactId>
 		</dependency>
 		<dependency>
 			<groupId>com.fasterxml.jackson.core</groupId>