Extract the `Scheduler` service to `backend-defaults` and deprecated the `backend-tasks` package

[camilaibs]

Context

Ref: https://github.com/backstage/backstage/issues/24493

Default service implementations should be provided from the backend-default package before we release a stable backend system version.

Description

The schedule service implementation, which is currently exported from the backend-tasks package, should be exported as a separate subpath from the backend-defaults package.

Acceptance criteria

• Extracted code should be re-exported from the new module in a old package deprecated.ts file
• Deprecation exports should be annotated with @deprecated and contain a comment explaining what to replace with
• Other packages' import statements should be updated to use bindings from the new package
• Documentation examples should be revisited use code from the new package instead

[freben]

WIP branch: https://github.com/backstage/backstage/tree/freben/tasks-plugin-api

[freben]

Discussion ongoing about what our package/export structure should be with all this