Update Windows installer license year to 2026

[KerimDemirkaynak]

This PR simply updates the copyright year in the Windows installer license text (Aegisub/packages/win_installer/license.txt) from 2020 to 2026 to reflect the current year.

[arch1t3cht]

Commenting on the InnoSetup language changes here because you closed #621 (was this intentional?): As long as there is no stable InnoSetup release containing the new official languages, it would probably be better to download Simplified and Traditional Chinese as unofficial languages from a pinned tag rather than from main, right?

[KerimDemirkaynak]

Commenting on the InnoSetup language changes here because you closed #621 (was this intentional?): As long as there is no stable InnoSetup release containing the new official languages, it would probably be better to download Simplified and Traditional Chinese as unofficial languages from a pinned tag rather than from main, right?

Yes, closing #621 was intentional. I was actually trying to push the commits from the yumatu branch to another branch, but I accidentally pushed them to the dev branch (which is this PR) instead. I've cleaned up the commit history here now. Also, I completely agree with your reasoning about waiting for a stable release. I might reopen it once a new stable InnoSetup version is out.