Update copyright year to 2025

[martinthonon]

Reminder for next year: find . -type f -exec sed -i 's/Copyright 2025/Copyright 2026/g' {} +

[ardocrat]

This is unusual practice, header is updating only on first file creation, example: https://github.com/cocos/google-game-sdk/blob/master/GameActivity/src/main/java/com/google/androidgamesdk/GameActivity.java

[martinthonon]

It's a follow up to this PR, but as Antiochp suggested, we should just get rid of these years. https://github.com/mimblewimble/grin/pull/3592

[ardocrat]

It was actual before indeed https://en.wikipedia.org/wiki/Copyright_notice#Reasons_to_include_an_optional_copyright_notice