main: say in the docs and the 11.0.0-preview changelog that EFCore.ComplexIndexes.PostgreSQL cannot be installed beside the net11 packages yet #322

Problem

docs/efcore.md points readers at EFCore.ComplexIndexes.PostgreSQL for exclusion constraints, GiST indexes and PostgreSQL 18 temporal keys over this package's range columns (from line 240 on). ComplexIndexes 5.4.x pins Npgsql.EntityFrameworkCore.PostgreSQL to [10.0.0, 11.0.0), so a net11.0 consumer of 11.0.0-preview.1 cannot restore it next to our EF package.

The repository already acts on this: test/consumer-smoke-test.sh skips the ComplexIndexes consumer on net11, and ComplexIndexesCrossTests is compiled out. A reader isn't told: neither docs/efcore.md nor the ## [11.0.0-preview.1] section of the changelog says so.

Found by the review of PR #292 on 2026-10-19.

Change

• One sentence at the top of the ComplexIndexes sections of docs/efcore.md: on the 11 line, EFCore.ComplexIndexes is succeeded by CodoMetis.EFCore.SchemaKit.PostgreSQL (Epic: CodoMetis.EFCore.SchemaKit 11.0.0 — the EF Core 11 successor EFCore.ComplexIndexes#37, decided 2026-10-19), which isn't published yet (SchemaKit 11.0.0-preview.1: the first slice of the new API, on EF Core 11 / Npgsql 11 under the new ids EFCore.ComplexIndexes#38). Until it is, use 10.x with EFCore.ComplexIndexes 5.x.
• A line in the 11.0.0-preview.1 changelog section, under what can break you.
• Both go before 11.0.0-preview.1 is published, and are replaced by main: re-enable the ComplexIndexes cross-tests and the joint smoke consumer against CodoMetis.EFCore.SchemaKit.PostgreSQL #333.