Fix CI NU1603 errors by pinning ppy.Veldrid.SPIRV transitive dependency

`ppy.osu.Framework 2026.421.1` declares a dependency on
`ppy.Veldrid.SPIRV (>= 1.0.15-gb268bf39ea)`, which is a winnerspiros fork
build that was never published to any NuGet feed. NuGet correctly
resolves to the nearest available version (`1.0.15-gb66ebf81d2` on
nuget.org) and emits warning NU1603. The Code Quality and Test CI jobs
use `-warnaserror`, so the warning becomes 22–66 build errors per job.

Fix: add an explicit `<PackageReference>` for `ppy.Veldrid.SPIRV` at a
published version in `osu.Game.csproj`. This makes the resolution
deterministic, silences NU1603, and flows transitively to every other
project in the solution. The pinned version is binary-compatible with
what the framework expects.

Android/iOS build-only jobs were unaffected because they don't use
`-warnaserror`.

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/1d1c5bf8-4c9c-40e3-8d0f-34b9cde1a32c

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game/osu.Game.csproj

@@ -39,6 +39,15 @@
     </PackageReference>
     <PackageReference Include="Realm" Version="20.1.0" />
     <PackageReference Include="ppy.osu.Framework" Version="2026.421.1" />
+    <!--
+      Explicitly pin `ppy.Veldrid.SPIRV` to a published version. `ppy.osu.Framework` 2026.421.1 was
+      built against `ppy.Veldrid.SPIRV 1.0.15-gb268bf39ea`, which is a winnerspiros fork build that
+      was never published to a public NuGet feed. Without this explicit reference NuGet falls back
+      to the nearest available version on nuget.org and emits NU1603, which `-warnaserror` (used by
+      both the `Code Quality` and `Test` CI jobs) turns into build errors. The published version
+      below is binary-compatible with what the framework expects.
+    -->
+    <PackageReference Include="ppy.Veldrid.SPIRV" Version="1.0.15-gb66ebf81d2" />
     <PackageReference Include="ppy.osu.Game.Resources" Version="2026.420.0" />
     <PackageReference Include="Sentry" Version="6.2.0" />
     <!-- Held back due to 0.34.0 failing AOT compilation on ZstdSharp.dll dependency. -->