Do not polyfill StringBuilder.MoveChunks on net11

[SimonCropp]

Bug

StringBuilderPolyfill.cs (StringBuilder.MoveChunks) 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 reflection-based polyfill was dead code and counted as a polyfilled API.

Fix

Guard changed to #if !NET11_0_OR_GREATER.

Coverage

• New MoveChunks_NotPolyfilledWhereBclHasIt test asserts via reflection that Polyfills.Polyfill does not declare MoveChunks(StringBuilder) when StringBuilder does. Fails on net11.0 before the fix; passes on net48, net10.0 and net11.0 after.
• The existing MoveChunks tests exercise the polyfill below net11 and the BCL on net11; Consume.cs already calls it on every TFM.

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

Note: apiCount.include.md will conflict with the File.OpenNullHandle PR on the net11.0 row; rerunning ApiBuilderTests after merging either resolves it.