feat: implement deprecation of legacy request methods

[runspired]

RFC https://rfcs.emberjs.com/id/0964-deprecate-legacy-request-support/

Misc

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

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

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()