Gradual expansion of kernel backward compatibility support

### Description

The current system is well-architected for multi-kernel support, with features like:

- Caching
- Modular compilation
- Relaxed `modpost` handling

These lay a strong foundation for extending support to older Linux kernel versions. The plan is to incrementally increase backward compatibility, focusing on stability and maintainability.

### Strategy

* **Start with stable LTS kernels** across v3.x to v6.x, where API changes are manageable.
* Carefully test and adapt for older versions (v2.6, early v3.x) only as needed.
* Use conditional compilation (`#ifdef`) for symbol/function differences in older kernels.
* Expand the CI matrix slowly (keeping feedback loops fast and logs readable) to include additional LTS kernels, starting from the newest going backwards.
* Document any compatibility fixes or patches needed per version.
* Optionally explore **v2.6.x** support for extreme backward compatibility (low priority).

This approach ensures we build towards broader compatibility without compromising stability or developer velocity.

The issue describes a strategy but not specific code changes. Start by examining the existing kernel module code, likely in the main .c files, to understand the current multi-kernel support architecture. Look for existing conditional compilation (#ifdef) patterns. The CI matrix configuration files (e.g., .github/workflows/) will show current kernel testing. 'Done' means the module successfully builds and passes tests for an additional, older LTS kernel version, with any necessary compatibility fixes documented.

Written by the indexing model from the issue text.