tracking: emberjs/rfcs#964 deprecate-legacy-request-support

[runspired]

Original Issue: #8873 RFC https://rfcs.emberjs.com/id/0964-deprecate-legacy-request-support/

The below represents the full check list of what we need to deprecate. Some may be done in follow up PRs.

Misc

• [ ] Deprecate missing relationship data member being treated as empty for sync relationships. #7584

Store APIs

We will consider moving the implementation of these into the legacy package later in case restoring behavior for longer is desired. But their presence in core is no longer wanted.

• [x] store.findRecord
• [x] store.findAll
• [x] store.query
• [x] store.queryRecord
• [x] store.saveRecord
• [x] store.getReference
• [x] store.modelFor (base impl)

Optional Store APIs when not explicitly registered from the legacy package

• [x] store.pushPayload
• [x] store.serializeRecord
• [x] store.adapterFor
• [x] store.normalize
• [x] store.serializerFor

Model APIs

We should consider how to allow restoring these when using the legacy package.

• [x] model.save
• [x] model.reload
• [x] model.destroyRecord
• [x] model.isReloading
• [ ] model.serializer
• [ ] model.adapterError
• [ ] model._createSnapshot
• [ ] model.errors

Relationships

All of the following would be deprecated if used in a non-LinksMode relationship:

• [ ] accessing an async belongsTo
• [ ] accessing an async hasMany
• [ ] HasManyReference.load()
• [ ] HasManyReference.reload()
• [ ] BelongsToReference.load()
• [ ] BelongsToReference.reload()