chore(sdk): exclude SkiaSharp from auto-update on 6.6 (hold 3.119.2 pin)

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Follow-up to #2158. That PR reverted SkiaSharp 3.119.4 → 3.119.2 on 6.6, but the Uno.Sdk.Updater would re-bump it on the next automated "Uno.Sdk Update" PR. This adds SkiaSharp to .github/sdk-update-exclude.json (the updater's --exclude-file, already used for SvgSkia / WinAppSdk) so the pin holds.

Remove the SkiaSharp exclude once 3.119.5 (mono/SkiaSharp#3798) ships or on the v4 move in 7.0.

Related to unoplatform/uno#23680
Related to unoplatform/uno.chefs#1750
Related to https://github.com/unoplatform/uno-private/issues/1957#issuecomment-4938024224

(Related to https://github.com/unoplatform/private/issues/1076)

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Why SkiaSharp is excluded from the auto-updater

SkiaSharp is held at 3.119.2 (the last iOS-safe version) on purpose. 3.119.4 ships its Apple assets targeting iOS 26.2 (net10.0-ios26.2), which are incompatible with iOS < 26.2 (Xcode < 26.2) builds → NuGet falls back to the platform-neutral net10.0 assembly with no native libSkiaSharp binding → iOS apps break (runtime DllNotFoundException: libSkiaSharp / MT0099 / NU1605).

Adding SkiaSharp to .github/sdk-update-exclude.json stops the Uno.Sdk.Updater from re-bumping it back to 3.119.4 in the automated "Uno.Sdk Update" PRs (same mechanism already used for SvgSkia / WinAppSdk).

Related:

• Framework-level root cause / tracking: SkiaSharp 3.119.4 (iOS 26.2 assets) breaks iOS <26.2 builds (canaries + consumer apps, e.g. Uno.Chefs) uno#23680
• Consumer-app repro (Chefs iOS): Chefs doesn't launch when built for net10.0-ios uno.chefs#1750
• Upstream SkiaSharp fix (corrects the Apple asset TFM 26.2 → 26.0): Fix Apple platform TFMs: use 26.0 for libraries, unversioned for apps mono/SkiaSharp#3798
• Backport request / status (not in our control): [BUG] SkiaSharp.NativeAssets.MacCatalyst 3.119.4-preview.1.1 SDK version set to 26.2 mono/SkiaSharp#4031 (comment)

When to remove this exclude (and let SkiaSharp move forward again): whichever comes first —

1. the SkiaSharp team ships a 3.119.5 backport of Fix Apple platform TFMs: use 26.0 for libraries, unversioned for apps mono/SkiaSharp#3798 (then bump to it), or
2. we move to SkiaSharp v4 (4.148.0+) — planned for Uno.Sdk 7.0 — which already carries the ios26.0 fix.

Until then, we deliberately hold the SkiaSharp bump at 3.119.2.