Add @Start and deprecate @Up

[eunomie]

Adds @Start, the Java marker for a service function, as part of the dagger up → dagger start rename (dagger/dagger#14218). The engine part is dagger/dagger#14328.

This change works on engines that predate start. @Start registers the function through Function.withUp, the same call @Up makes. Engines have had withUp since v0.20.4.

• @Start (io.dagger.module.annotation.Start) marks a function as a service for dagger start.
• @Up keeps working. It is @Deprecated, and its Javadoc points at @Start. Modules that use it build as before; javac adds a deprecation note.
• The annotation processor treats either annotation as a service marker. Its own lookup of @Up suppresses the deprecation warning, because the processor is compiled inside every generated module.
• A new e2e check, e-2-e:service-marker-check, generates a module with a @Start function, an @Up function, and an unmarked function that returns a Service. It loads the module and asserts through Module.services that only the first two are services.

Verification

• engine-e-2-e:sdk-contract-check (all 8 e-2-e:* checks) and engine-e-2-e:dev-sdk-check pass at e853f4e. Both run inside an engine built from dagger/dagger 8fd9b22b5416, the commit engine-dev is pinned to. That engine has dagger up and no start, so the passing service-marker-check shows that @Start works on an engine that predates start.
• Negative control: with the processor's @Start branch removed, service-marker-check fails with a @Start function should be a service; services: up-marked.

Notes

• A module gets @Start when dagger generate vendors the new SDK source into it. Until then its vendored copy has only @Up, which keeps working.
• Modules on the engine's builtin java runtime use dagger/dagger sdk/java, which gets @Start in Rename dagger up to dagger start dagger#14328.
• This repository has no tags and no changelog. Workspaces install it from its default branch, so merging releases it; a workspace that locked an earlier commit gets it when it refreshes its lock. This description is the record of the change.