Error with tomcat when deploying Google App Engine application - google-app-engine

Question

I'm following this tutorial to deploy my API on GAE with a SQL Cloud DB and it's been several hours now that I'm testing all the possibilities but I still encounter several errors that I can't fix. When I exclude the tomcat dependency I get the error :
java.lang.NoClassDefFoundError: javax/servlet/http/HttpServletMapping
at org.springframework.test.context.web.ServletTestExecutionListener.setUpRequestContextIfNecessary(ServletTestExecutionListener.java:206) ~[spring-test-5.3.4.jar:5.3.4]
at org.springframework.test.context.web.ServletTestExecutionListener.prepareTestInstance(ServletTestExecutionListener.java:132) ~[spring-test-5.3.4.jar:5.3.4]
at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:244) ~[spring-test-5.3.4.jar:5.3.4]
at org.springframework.test.context.junit.jupiter.SpringExtension.postProcessTestInstance(SpringExtension.java:138) [spring-test-5.3.4.jar:5.3.4]
at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$invokeTestInstancePostProcessors$6(ClassBasedTestDescriptor.java:350) [junit-jupiter-engine-5.7.1.jar:5.7
When I remove this exclusion from my pom.xml :
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</exclusion>
</exclusions>
the app works fine locally and connect well to my SQL Cloud DB but when I deploy with flex env I get those error on myproject.region.r.appspot.com :
Error: Server Error
The server encountered an error and could not complete your request.
Please try again in 30 seconds.
and in cloud console Error reporting :
java.lang.RuntimeException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Invocation of init method failed; nested exception is javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory; nested exception is org.hibernate.exception.GenericJDBCException: Unable to open JDBC Connection for DDL execution
at
org.eclipse.jetty.annotations.ServletContainerInitializersStarter.doStart (ServletContainerInitializersStarter.java:68)
at
org.eclipse.jetty.util.component.AbstractLifeCycle.start (AbstractLifeCycle.java:68)
at
org.eclipse.jetty.servlet.ServletContextHandler.startContext (ServletContextHandler.java:330)
at
org.eclipse.jetty.webapp.WebAppContext.startWebapp (WebAppContext.java:1406)
at
com.google.apphosting.runtime.jetty9.AppEngineWebAppContext.startWebapp (AppEngineWebAppContext.java:175)
at
org.eclipse.jetty.webapp.WebAppContext.startContext (WebAppContext.java:1368)
at
org.eclipse.jetty.server.handler.ContextHandler.doStart (ContextHandler.java:778)
Thank you.