[go-server] Return Router interface from controller constructor.

[Jesse0Michael]

Update the go server api controller template to return the Router interface instead of the api specific router.
The Router interface type has the Routes function, which is what the generated NewRouter function needs. (not the api specific interface).

The change added in #4038 would cause the following error since it was working with the wrong interface type

./main.go:38:36: cannot use PetApiController (type petstoreserver.PetApiRouter) as type petstoreserver.Router in argument to petstoreserver.NewRouter:
        petstoreserver.PetApiRouter does not implement petstoreserver.Router (missing Routes method)

@antihax @bvwells @grokify @kemokemo @bkabrda

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before .
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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[wing328]

@Jesse0Michael thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569