Use single OIDC session auth method instead of per-provider values

The OIDC provider is already tracked in iam_oidc_states.provider, so
there is no need for provider-specific session auth methods (GOOGLE,
MICROSOFT). Replace them with a single OIDC auth method.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/coredata/migrations/20260319T150000Z.sql

@@ -1,5 +1,4 @@
-ALTER TYPE session_auth_method ADD VALUE 'GOOGLE';
-ALTER TYPE session_auth_method ADD VALUE 'MICROSOFT';
+ALTER TYPE session_auth_method ADD VALUE 'OIDC';
 
 CREATE TYPE iam_oidc_provider AS ENUM (
     'GOOGLE',