docs: update Ruby compatibility guide

[realityforge]

Summary

• update the compatibility baseline to the current post-v0.9.8 development state
• clarify the primary workflows and migration impact for human readers
• document the removed setup command and the new sync and completion commands
• clarify upgrade-config --no-commit behavior

Validation

• bazel run @rules_go//go -- fmt ./...
• git diff --exit-code
• bazel test //...
• bazel run @rules_go//go -- vet ./...
• bazel run @rules_go//go -- run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.0 run
• bazel test //integration/...
• git diff --check