auth: collapse provider routing to OIDC constants, drop v1 (COR-393)

The v1 provider described the retired single-host world where entire.io was its own login server (homegrown device-code path, no STS). Everyone has been on the v2 OIDC surface since split-host became the default, so the version routing table, the ENTIRE_AUTH_PROVIDER_VERSION override, the split-host auto-detect (api.IsSplitHost), the process-wide sync.Once singleton, and the SetProviderForTest seam all reduce to four package constants.