Update Spring Boot release, adding Gradle build and cleanup

- Update Spring Boot release, Checkstyle, Mysql.
- Formatting pom.xml with sortpom-maven-plugin.
- Rename README to standard file name.
- Adding GitHub Action for Gradle.

src/test/java/org/springframework/samples/petclinic/MySqlIntegrationTests.java

@@ -46,7 +46,7 @@ class MySqlIntegrationTests {
 
 	@ServiceConnection
 	@Container
-	static MySQLContainer<?> container = new MySQLContainer<>("mysql:9.0");
+	static MySQLContainer<?> container = new MySQLContainer<>("mysql:9.1");
 
 	@LocalServerPort
 	int port;

src/test/java/org/springframework/samples/petclinic/MysqlTestApplication.java

@@ -36,7 +36,7 @@ public class MysqlTestApplication {
 	@Profile("mysql")
 	@Bean
 	static MySQLContainer<?> container() {
-		return new MySQLContainer<>("mysql:9.0");
+		return new MySQLContainer<>("mysql:9.1");
 	}
 
 	public static void main(String[] args) {

src/test/java/org/springframework/samples/petclinic/vet/VetTests.java

@@ -18,7 +18,6 @@ package org.springframework.samples.petclinic.vet;
 import org.junit.jupiter.api.Test;
 import org.springframework.util.SerializationUtils;
 
-import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**