chore: update ktsu.Sdk to 2.21.1 [patch]

[matt-edmondson]

Bumps ktsu.Sdk and its variants to 2.21.1 in global.json. File headers are migrated to the single line from COPYRIGHT.md, which is what the SDK writes into .editorconfig as file_header_template and IDE0073 enforces. Updates Color.Palette.* to Palette.*, and migrates SearchBox to the SearchBoxOptions API for ktsu.ImGui.Widgets 3.3.9.

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[matt-edmondson]

Closing as obsolete — superseded by main.

This PR was part of the org-wide chore/sdk-2.21.1-lf-migration sweep from 2026-08-11. Every change it carries has since landed on main through other routes:

• ktsu.Sdk 2.21.1 bump — main is now on a later SDK (2.25.0+), so the global.json change is a downgrade.
• License-header migration — the one-line // Copyright (c) 2023-2026 ktsu-dev contributors header is already on main.
• .gitattributes LF normalization — already on main, byte-identical to this branch.
• .editorconfig / .runsettings — already updated on main.
• Any API adaptations — already present on main.

The PR is also in a conflicting state and would need a full rebase to produce a diff that is, by then, empty. Nothing here is worth recovering.