feat: deprecate create_or_update in favour of upsert

The `create_or_update` method has been deprecated and replaced by
`upsert` for clarity and consistency with common database terminology.

This commit also removes the file `src/entity.rs` which has been
forgotten in earlier commits and was no longer part of Georm.

README.md

@@ -557,7 +557,7 @@ Post::find(executor, &post_id).await?;
 // Mutation operations
 post.create(executor).await?;
 post.update(executor).await?;
-post.create_or_update(executor).await?;
+post.upsert(executor).await?;
 post.delete(executor).await?;
 Post::delete_by_id(executor, &post_id).await?;