Do not polyfill File.OpenNullHandle on net11

[SimonCropp]

Bug

File.OpenNullHandle was guarded by #if !NET12_0_OR_GREATER, but the method ships in net11 (present in the Microsoft.NETCore.App.Ref 11.0.0-rc.1 pack pinned by global.json, absent from 10.0.12). On net11 the polyfill was dead code and counted as a polyfilled API. Consume.cs hid this by only calling it under #if !NET11_0_OR_GREATER.

Fix

• Guard changed to #if !NET11_0_OR_GREATER (the block also holds HardLinkHelper/NullDeviceHelper, which are only needed below net11).
• Consume.cs now calls File.OpenNullHandle() on every TFM.

Coverage

• New FilePolyfillTests.OpenNullHandle_NotPolyfilledWhereBclHasIt asserts via reflection that Polyfills.Polyfill does not declare OpenNullHandle when System.IO.File does. Fails on net11.0 before the fix; passes on net48, net10.0 and net11.0 after.
• The existing OpenNullHandle test exercises the polyfill below net11 and the BCL on net11.
• Consume built for net461, net10.0 and net11.0.

Split files and API counts regenerated (net11.0: 58 -> 57; src/Split/net11.0/FilePolyfill.cs is now empty and removed).