* Change the return type of a file back to a pointer
* Change the api template to handle not double pointer-ing return types of os.File
* Fix unit tests
* Couple more unit test fixes
* Changes manually cherry-picked (for the most part) from https://github.com/OpenAPITools/openapi-generator/pull/12685/files
* Examples updated post changes
* Missed a change in the mustache template
* Update examples after last fix
* Missed dereference for required files
* Update unit tests
* Missed another test case update
* `f := *f` isn't quite the same as `*f, err = ...`
According to the [documentation](https://openapi-generator.tech/docs/generators/go-server) the go-server generator should support an `addResponseHeaders` option but a template uses an `addResponseHeader` option in some places which this PR fixes.
Closes#9795
Signed-off-by: Max Goltzsche <max.goltzsche@gmail.com>
