backend-main-fehlerbehebung

• Modify the main-branch by..

• changing .misc.xml -> JDK22 to JDK21
• Backend[movie-app] Module to Backend[Backend], so there are no conflicts in the naming
• adding .env to gitignore, so no sensitive data gets leaked
• changing name of the MovieAppApplication to MovieApplication
• changing pom.xml project name from "movie-app" to "filmdatenbank"
• change the description of the project to "API´s related to movies"