Update vector search docs for VectorSearch() API changes (dotnet/efcore#38075)

[Copilot]

The topN parameter was removed from VectorSearch() in dotnet/efcore#38075. Users now compose with standard LINQ operators instead, and SQL Server generates TOP(...) WITH APPROXIMATE rather than TOP_N inside VECTOR_SEARCH().

Changes

• vector-search.md: Replaced all VectorSearch(..., topN: n) calls with .OrderBy(r => r.Distance).Take(n) pattern, updated generated SQL examples, updated hybrid search example (both C# and SQL)
• whatsnew.md (EF Core 11): Same API update for the what's new code snippet and description

New pattern

// Before
var blogs = await context.Blogs
    .VectorSearch(b => b.Embedding, embedding, "cosine", topN: 5)
    .ToListAsync();

// After
var blogs = await context.Blogs
    .VectorSearch(b => b.Embedding, embedding, "cosine")
    .OrderBy(r => r.Distance)
    .Take(5)
    .ToListAsync();

Generated SQL now uses SELECT TOP(@p) WITH APPROXIMATE instead of TOP_N = @p inside the VECTOR_SEARCH() TVF.