Bump the license year to 2026

What is the new feature

The license header in each file now has the correct year.

Also, the version of ICONMATH is updated in CMakeLists.txt

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

You are not supposed to merge this request by yourself, the maintainers of libiconmath take care of this action!