Fix symbol/string :errors/'errors' confusion in GQL client

What does this MR do and why?

⚠ Let's wait until the beginning of %16.9 with the merge to de-risk.

We should use response[:errors] (with symbol) in cases where we access the hash we constructed in rescue.
We should use response['errors'] (with string) when the actual errors comes from the client.

We need to consolidate that to use the uniform access.

Originally discovered and discussed here: !140980 (comment 1720061858)

See https://gitlab.com/gitlab-org/gitlab/blob/4e18e71ae0682d3bc1eed071863b3b210841e2de/ee/lib/gitlab/subscription_portal/clients/rest.rb#L78 for the hash format in case of rescue.

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How to set up and validate locally

I did !140980 (merged) while working on that change and then turned off the local CustomersDot instance.