General update of the code generator

[jcomellas]

The following changes have been made:

• Replace calls to any function that had been deprecated up to Elixir 1.9
• Upgrade all dependencies to their latest version (this required adapting the calls to Poison.Parser.parse!/2)
• Modification of the code generators so that no warnings or errors are shown when compiling the generated code.

Practically no effort has been made to format the code according the current Elixir formatting standards. This will be submitted in a followup PR.

The code generation was tested using the Go AWS SDK v1.25.14.

[jcomellas]

@tsantero This might interest you.

[jadeallenx]

Thanks