Write the 2.0 migration guide #404

2.0 needs a migration guide in the docs, as modern-di has for each major (docs/migration/to-4.x.md there). The release notes will carry a "Breaking changes" list, but a guide lets users go through what to change in their code and config one section at a time, with before and after where it helps.

Work

• Add docs/migration/to-2.x.md, "Migration guide: upgrading to lite-bootstrap 2.x", and a "Migration" section in mkdocs.yml nav. Link it from README.md (absolute link, per AGENTS.md) and from the 2.0.0 release notes.
• One section per change a user may have to act on. Each says what changed, what breaks or behaves differently, and what to do. Group them, for example: removed names and config fields, new defaults, dependency floors and caps, logging, Sentry, OpenTelemetry, Prometheus, health checks, per framework.
• Source: the "Breaking changes" section of every PR merged since 1.16.0. As of today that is feat(sentry)!: support sentry-sdk 3 alongside 2.x and raise the floor to 2.70 #359 to Cap sentry-sdk below 3 and remove the 3.x code paths #379:
  • feat(sentry)!: support sentry-sdk 3 alongside 2.x and raise the floor to 2.70 #359, feat(sentry)!: leave release-health sessions off and start log breadcrumbs at WARNING #360, feat: capture Litestar errors without LitestarIntegration while Sentry tracing is off #361, Report FastAPI and FastMCP errors without StarletteIntegration while tracing is off #362, Leave sentry-sdk's auto-enabling integrations off while Sentry tracing is off #363, Honour OTEL_SERVICE_NAME and send service_version as the Sentry release #373, Cap sentry-sdk below 3 and remove the 3.x code paths #379: Sentry
  • Default opentelemetry_exclude_spans to ["receive", "send"] and support it on FastMCP #364, Record HTTP metrics once when both OpenTelemetry metrics and Prometheus are configured #365, Default opentelemetry_generate_health_check_spans to False #366, Raise the OpenTelemetry floor to 1.42 / 0.63b0 and delete its shims #368, Default opentelemetry_insecure to None so OTEL_EXPORTER_OTLP_INSECURE takes effect #372: OpenTelemetry
  • Remove the names and config fields deprecated in 1.x #367: names and fields deprecated in 1.x removed
  • Default service_debug to False and keep Litestar's explicit AppConfig.debug #369: service_debug default
  • Return 503 from an unhealthy health check #370: 503 from an unhealthy health check
  • Raise the base litestar floor to 2.22 and delete the plugin probe #371, Raise the fastmcp floor to 4.0 and bound it below 5 #374: Litestar and FastMCP floors
  • FastStream Prometheus: use the default registry and make the metrics path opt-in #375: FastStream Prometheus default
  • Make BaseBootstrapper generic over its config #376: BaseBootstrapper generic over its config
  • Write structlog lines through a plain StreamHandler and remove logging_buffer_capacity and logging_flush_level #378: logging buffer removed
• Write it last on the milestone, after the other 2.0.0 issues are merged, and add their breaking changes too.
• Check every config example in the guide against the code (just docs-build, and run the snippets where possible).