stjm: move .NET 11 from the RC1 pin to GA (2026-11-10) #206

Egil.SystemTextJson.Migration multi-targets net10.0;net11.0 since #199, built against the .NET 11 RC1 SDK under the go-live licence. Once .NET 11 ships (GA 2026-11-10), drop the prerelease pins.

Changes

• .github/workflows/egil-systemtextjson-migration-ci.yml: replace 11.0.100-rc.1.26425.128 with 11.0.x in every setup-dotnet step (four occurrences) and rewrite the SDK band policy comment above jobs: so it no longer mentions the RC1 pin.
• Egil.SystemTextJson.Migration/global.json: set allowPrerelease to false.
• Re-run the benchmarks on the GA runtime and refresh docs/perf/*.md plus the README perf table with scripts/update-perf-docs.ps1 (the current numbers state 11.0.0-rc.1.26425.128).
• Re-run the full suite on both target frameworks in Release and confirm the build stays warning-free; the JsonTypeClassifier and JsonUnion APIs used by UnionCaseRouting and JsonMigratableUnionTypeClassifier were RC1 shapes, so a GA rename would surface here.
• Re-check [API Proposal]: Expose JsonConverter.CanHaveMetadata dotnet/runtime#118900 (JsonConverter.CanHaveMetadata). If it became public in GA, the [JsonPolymorphic] limitation in docs/recipes/polymorphism.md can be revisited; otherwise leave the recipe as is.

Commit as build(stjm): … [skip notes] unless the runtime changes force a consumer-visible fix.