Which official LB3 components are not planned in LB4

[bajtos]

This is a follow-up for #3718 and #3922.

Write content for docs/site/migration/not-planned.md. Review the list of official LB3 components in "Understanding differences"), pick those that we are not planning to bring over to LB4, and explain that we are not planning to upgrade them to LB4 and if possible, then advise users how to upgrade on their own (e.g. how they can implement them themselves, or point to 3rd-party/community alternatives, etc.).

/cc @dhmlau

Acceptance criteria

Let's focus on the 3 listed below for this issue. Follow up tasks can be created for any we missed.

• loopback-compontent-storage
• loopback-component-push
• Offline synchronization