Prevent Thymeleaf to be configured with Thymeleaf 3

Spring Boot does not support Thymeleaf 3 yet. This commit prevents the
auto-configuration to activate if Thymeleaf 3 is available on the
classpath.

Closes gh-5371

Author: snicoll

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/thymeleaf/ThymeleafAutoConfiguration.java

@@ -43,6 +43,7 @@
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnJava;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
 import org.springframework.boot.autoconfigure.template.TemplateLocation;
@@ -68,6 +69,7 @@
 @Configuration
 @EnableConfigurationProperties(ThymeleafProperties.class)
 @ConditionalOnClass(SpringTemplateEngine.class)
+@ConditionalOnMissingClass("org.thymeleaf.templatemode.TemplateMode")
 @AutoConfigureAfter(WebMvcAutoConfiguration.class)
 public class ThymeleafAutoConfiguration {