auth/backends: Fix cache TTL computation for OIDC profile

The cache TTL for storing an OIDC profile must be computed from the
access token renewal date (iat field in decoded token) and not from
the OIDC session opening date (auth_time field in decoded token).

Previous implementation was computing a negative TTL (clamped to 0)
once the first issued refresh token was expired and thus the
authentication process was then failing.