Add missing class condition on thymeleaf-spring5

This commit adds an extra check for the presence of thymeleaf-spring5
library on the classpath. ThymeleafAutoConfiguration is now only
considered if both thymeleaf-spring5 and thymeleaf jars are present.

Closes gh-16341

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

@@ -77,7 +77,7 @@ import org.springframework.web.servlet.resource.ResourceUrlEncodingFilter;
  */
 @Configuration
 @EnableConfigurationProperties(ThymeleafProperties.class)
-@ConditionalOnClass(TemplateMode.class)
+@ConditionalOnClass({ TemplateMode.class, SpringTemplateEngine.class })
 @AutoConfigureAfter({ WebMvcAutoConfiguration.class, WebFluxAutoConfiguration.class })
 public class ThymeleafAutoConfiguration {