fix: Make amr claim an array to match the OIDC spec

[svrakitin]

This change makes amr claim follow the spec and be of an array type.

See: https://openid.net/specs/openid-connect-core-1_0.html#IDToken

My team noticed that when trying to use amr claim with Hydra.

Will follow up with Hydra PR to add support for amr claim as it is being dropped right now if used. Related issue: ory/hydra#1756

Previous change: #401

Checklist

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Further comments

This is breaking change, migration path is to wrap single values into the []string slice and use a different field.