Make OAuth2 issuer configurable (#35915)

The new (correct) behavior breaks the old (incorrect) logins.

Add a config option to support legacy "issuer".

Fix #35830

modules/setting/oauth2.go

@@ -96,6 +96,7 @@ var OAuth2 = struct {
 	InvalidateRefreshTokens    bool
 	JWTSigningAlgorithm        string `ini:"JWT_SIGNING_ALGORITHM"`
 	JWTSigningPrivateKeyFile   string `ini:"JWT_SIGNING_PRIVATE_KEY_FILE"`
+	JWTClaimIssuer             string `ini:"JWT_CLAIM_ISSUER"`
 	MaxTokenLength             int
 	DefaultApplications        []string
 }{