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: Ia1460e85e418b75844abae7d5cce282cd1f66d7f
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/OWNERS b/OWNERS
index 944b1b1..325d94f 100644
--- a/OWNERS
+++ b/OWNERS
@@ -33,7 +33,7 @@
 
 owners:
 - anoo@us.ibm.com
-- gmills@linux.vnet.ibm.com
+- gunnar@gmills.xyz
 - mine260309@gmail.com
 
 reviewers:
@@ -47,7 +47,7 @@
   email: anoo@us.ibm.com
   discord: anoo
 - name: Gunnar Mills
-  email: gmills@linux.vnet.ibm.com
+  email: gunnar@gmills.xyz
   discord: GunnarM
 - name: Lei Yu
   email: mine260309@gmail.com