Bump version to 1.26.0 (#27955)
### Description
Increase version number to 1.26.0. The rel-1.25.0 release branch has
been cut.
### Changes
- VERSION_NUMBER: 1.25.0 → 1.26.0
- ORT_API_VERSION: 25 → 26 (header + C API struct rename)
- Python, JS, docs version strings updated via update_version.py
- C# NativeTrainingMethods ORT_API_VERSION: 23 → 26
- samples/cxx/README.md example paths updated
- docs/Versioning.md example updated
### Motivation and Context
Per release process: bump main branch version immediately after cutting
the release branch.
