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',

pkg/coredata/session.go

@@ -57,8 +57,7 @@ const (
 	AuthMethodMagicLink AuthMethod = "MAGIC_LINK"
 	AuthMethodPassword  AuthMethod = "PASSWORD"
 	AuthMethodSAML      AuthMethod = "SAML"
-	AuthMethodGoogle    AuthMethod = "GOOGLE"
-	AuthMethodMicrosoft AuthMethod = "MICROSOFT"
+	AuthMethodOIDC      AuthMethod = "OIDC"
 )
 
 func NewRootSession(identityID gid.GID, method AuthMethod, duration time.Duration) *Session {