[Go] [Client] Fix #4571: Allow handling of files in responses

[BabisK]

Decode method in client is able to handle files.

Requires Go 1.16. This is reflected on the change in workflows and the go.mod created in the tests.

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [x] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This PR changes the client.go generated to handle files in the body of the response.

The code changed is in client.mustache, most other changes are generated code. Also updated go to 1.16 as this contains the API required. Thus also created go.mod in the tests folder.

Should fix #4571