[REQ] Deprecating Unreal Engine 4 support

Is your feature request related to a problem? Please describe.

I am the original author and maintainer of the cpp-ue4 template.
For maintenance purposes, i'd like to deprecate Unreal 4 support and focus only on forward compatible Unreal 5 support.

Describe the solution you'd like

There are several ways to approach this and I'd want the opinion and approval of the maintainers of openapi-generator to move forward.

1. Create a new template cpp-ue5, and deprecate cpp-ue4. I will no longer maintain cpp-ue4
2. Add a new template cpp-ue, with explicit guarantees of compatibility with Unreal Engine versions. How far back do we want such compatibility?
3. Keep maintaining both in cpp-ue4, however the complexity and areas that require polyfill will keep increasing. The generated code is also less readable.
4. Create an option in the generator and try to resolve these conflicts by picking different mustache files or switching on the unreal engine version. This is also a viable and elegant way to deal with the issue, but the generator name is still cpp-ue4

Additional context

On average, Unreal releases 3 major versions per year. Unreal themselves on their marketplace only support the latest version and 3 version back for code plugins. I think a year of support is reasonable in this case.

Start by reviewing the cpp-ue4 template and the 13-comment discussion. Compare the four proposed approaches—cpp-ue5, cpp-ue, continued dual support, or a generator option—and determine which path maintainers approve. Done means an agreed compatibility and deprecation plan is documented.