Use "real" symbol instead of string for yieldableSymbols

[sly7-7]

Upgrading my app to ember-data 4.7, I see these kind of deprecations

 Accessing __ec_cancel__ on this PromiseArray is deprecated. The return type is being changed from PromiseArray to a Promise. 

It seems like this is on purpose on ED side, see https://github.com/emberjs/data/pull/8025/files#diff-6a82e4ae6a1de9585e8aea31c2a19871952d300d350811678ef0718879f8bddbR57

I was wondering if using Symbol here instead of string would be better, and also resolve this deprecation, since it would pass this check:

https://github.com/emberjs/data/blob/master/packages/store/addon/-private/proxies/promise-proxies.ts#L128

[bartocc]

chiming in to see if there's any progress on this

[TSenter]

I would also like to know what the status of this is - I'm preparing to upgrade an addon when the first LTS version of Ember 5 is released, and this is one of the big roadblocks.