Remove the type for the `Object#_objectId`

[kraenhansen]

What, How & Why?

This closes #4820

☑️ ToDos

• [ ] 📝 Changelog entry
• [ ] Migrate this over to the _objectKey introduced on master (after v11 has been rebased):
  • [ ] tests/js/linkingobjects-tests.js#L219-L230
  • [ ] integration-tests/tests/src/tests/class-models.ts#L114
  • [ ] lib/extensions.js#L78
• [x] 📝 Compatibility label is updated or copied from previous entry
• [x] 🚦 Tests
• [x] 🔀 Executed flexible sync tests locally if modifying flexible sync
• [x] 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• [x] 📱 Check the React Native/other sample apps work if necessary
• [x] 📝 Public documentation PR created or is not necessary
• [x] 💥 Breaking label has been applied or is not necessary

[kraenhansen]

LGTM. I would approve if this wasn't my own PR 🤦

[takameyer]

The tests seem to not be getting the _objectKey changes. I'll have to look into this before merging