Document DbQueryConcurrencyException breaking change for split queries (EF Core 11)

[Copilot]

EF Core 11 (dotnet/efcore#38676) changes split query behavior: instead of silently discarding child collections when concurrent modifications cause out-of-order rows, EF now throws DbQueryConcurrencyException.

Changes to ef-core-11.0/breaking-changes.md

• New Low row in summary table: Split queries now throw when concurrent modifications are detected
• New section with old/new behavior, rationale, and three mitigations:
  • Retry on DbQueryConcurrencyException (transient)
  • Wrap in snapshot/serializable transaction
  • Switch to AsSingleQuery()

// EF Core 11: throws instead of silently returning empty Posts
catch (DbQueryConcurrencyException)
{
    // concurrent modification detected — retry or use a transaction
}