[docs]: Update server/README.md with alternative to simapp.MakeEncodingConfig()

[clevinson]

reference: see comment#7681 (comment).

In general, this sample code should be rewritten to not use EncodingConfig.

[amaury1093]

For the time being, maybe a solution could be just a comment to tell users to implement their own EncodingConfig instead of using MakeTestEncodingConfig

[robert-zaremba]

I think it's fine to use the MakeTestEncodingConfig. We just need to make sure that it's used only once. Basically, a user can use it ONLY:

• in tests
• when creating a new app

In all other places, users must use app.AppCodec