Bump skia to milestone 120

[mattleibow]

Bump Skia to Milestone 120

Updates the Skia submodule from m119 to m120.

Changes

• externals/skia: points to dev/update-skia-m120 branch (Update skia to milestone 120 skia#173)
• SkiaApi.generated.cs: regenerated — adds sk_surface_draw_with_sampling (new upstream function, wrapper deferred)
• VERSIONS.txt: milestone 120, soname 120.0.0, assembly 3.120.0.0, nuget 3.120.0
• cgmanifest.json: chrome_milestone 120, updated commit hashes

Breaking changes in Skia m120

Change	Impact	Fix
GrDirectContext::MakeGL() removed	C API compile failure	Updated to GrDirectContexts::MakeGL()
GrDirectContext::MakeVulkan() deprecated	Compiles but deprecated	Proactively updated to GrDirectContexts::MakeVulkan()
SkTypeface::UniqueID() static removed	None (not in C API)	Removed orphaned impl
SkFont typeface methods → private	None (not in C API)	No action
GrBackendSemaphore::initGL/glSync removed	None (not in C API)	No action

Verification

• ✅ Native build (macOS ARM64): 0 errors
• ✅ C# build: 0 errors
• ✅ Tests: 5374 total, 0 failed, 25 hardware skips

Companion PR

• Update skia to milestone 120 skia#173 — must be merged first

Merge sequence

1. Merge Update skia to milestone 120 skia#173 → squash creates new SHA on skiasharp
2. Update submodule pointer in this PR to the new SHA
3. Then merge this PR