fix(web): remove vulnerable SixLabors.ImageSharp packages

[eduardosmaniotto]

Removes the SixLabors.ImageSharp / SixLabors.ImageSharp.Drawing 2.1.11
dependencies, which carry known vulnerabilities (NU1902/NU1903 on every
build). Terrain rendering now uses a small dependency-free PNG encoder
(SimplePngEncoder, 8-bit RGB) plus ToPng() / ToPngDataUrl() extensions.
Rendered output is pixel-identical (black / gray safezone / green walkable).

Changes

• Deleted the duplicated Map-local GameMapTerrainExtensions; single
  implementation in Web.Shared with named SideLength / color constants
• MapEditor and ExitGatePicker hold a data-URL string instead of
  Image<Rgba32> (dispose bookkeeping removed; fixes an undisposed image
  in ExitGatePicker)
• GameMapInfoExtensions render cache (ConcurrentDictionary per map
  number) unchanged, only the render body swapped
• Package references removed from Web.Shared, Web.Map, AdminPanel
  and Directory.Packages.props

Verification

• dotnet build Shared / Map / AdminPanel: 0 NU190x, 0 errors
• Generated PNG validated structurally (signature, IHDR 256×256 RGB,
  chunk CRCs, IDAT size) with correct walk-green pixels
• dotnet test MUnique.OpenMU.Web.Tests: 200/200 pass
• Manual smoke: open map editor and exit-gate picker, confirm terrain
  renders; check live map tiles