Add serialVersionUID to generated classes that implement Serializable

[rolevinks]

We use the maven-compiler-plugin in our project, and this gives us warnings on generated classes that implement Serializable.

serializable class [...] has no definition of serialVersionUID

There is a simple, solution, to add this to the template:

    @java.io.Serial
    private static final long serialVersionUID = 1L;

Can this be implemented? I would do it myself, but when I check out and build the project, a lot of tests fail because of an UnableToDeleteDirectoryException.

Unable to delete directory
com.kobylynskyi.graphql.codegen.model.exception.UnableToDeleteDirectoryException: Unable to delete directory

Caused by: java.nio.file.FileSystemException: build\generated\com\kobylynskyi\graphql\test1\CreateEventMutationResolver.java: The process cannot access the file because it is being used by another process
	at java.base/sun.nio.fs.WindowsException.translateToIOException(WindowsException.java:92)
	at java.base/sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:103)
	at java.base/sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:108)
	at java.base/sun.nio.fs.WindowsFileSystemProvider.implDelete(WindowsFileSystemProvider.java:274)
	at java.base/sun.nio.fs.AbstractFileSystemProvider.delete(AbstractFileSystemProvider.java:105)
	at java.base/java.nio.file.Files.delete(Files.java:1146)
	at com.kobylynskyi.graphql.codegen.utils.Utils.deleteDir(Utils.java:161)
	... 101 more 

[rolevinks]

Is resolved with #811