LemonLDAP::NG: Set username claim name in accordance with specs

Spec: https://openid.net/specs/openid-connect-core-1_0.html#StandardClaims
Change-Id: I86831e83878261f8c364f43c3d3bce576453562e

src/main/java/com/googlesource/gerrit/plugins/oauth/LemonLDAPOAuthService.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/oauth/LemonLDAPOAuthService.java b/src/main/java/com/googlesource/gerrit/plugins/oauth/LemonLDAPOAuthService.java
index f1a0dbe..e4e6723 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/oauth/LemonLDAPOAuthService.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/oauth/LemonLDAPOAuthService.java
@@ -90,7 +90,7 @@
         throw new IOException("Response doesn't contain 'user' field" + jsonObject);
       }
       JsonElement id = jsonObject.get("sub");
-      JsonElement username = jsonObject.get("username");
+      JsonElement username = jsonObject.get("preferred_username");
       JsonElement email = jsonObject.get("email");
       JsonElement name = jsonObject.get("name");
       return new OAuthUserInfo(