OWNERS: Update Gunnar's email address

I have multiple email addresses in my Gerrit account but as
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/69506 shows my Preferred
email has to match the OWNERS file, do that.

More discussion in Discord at
[1] https://discord.com/channels/775381525260664832/817151035197358081/1209585278025601094

Tested: With https://gerrit.openbmc.org/c/openbmc/bmcweb/+/69549 my +2
means something again.

Change-Id: I091a5cc00acff4c268a86c6c9c79f705992738af
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/OWNERS b/OWNERS
index aca6e4b..c280368 100644
--- a/OWNERS
+++ b/OWNERS
@@ -37,7 +37,7 @@
 reviewers:
 - andrew@codeconstruct.com.au
 - anoo@us.ibm.com
-- gmills@linux.vnet.ibm.com
+- gunnar@gmills.xyz
 - yulei.sh@bytedance.com
 
 matchers:
@@ -57,7 +57,7 @@
   email: andrew@codeconstruct.com.au
   discord: arj
 - name: Gunnar Mills
-  email: gmills@linux.vnet.ibm.com
+  email: gunnar@gmills.xyz
   discord: GunnarM
 - name: Jeremy Kerr
   email: jk@ozlabs.org