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: I97d75054c68674ed5342df983e338ea524f60412
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/OWNERS b/OWNERS
index aa5308f..47729ba 100644
--- a/OWNERS
+++ b/OWNERS
@@ -33,7 +33,7 @@
 
 owners:
 - anoo@us.ibm.com
-- gmills@linux.vnet.ibm.com
+- gunnar@gmills.xyz
 
 reviewers:
 - ojayanth@in.ibm.com
@@ -45,7 +45,7 @@
   email: anoo@us.ibm.com
   discord: anoo
 - name: Gunnar Mills
-  email: gmills@linux.vnet.ibm.com
+  email: gunnar@gmills.xyz
   discord: GunnarM
 - name: Jayanth Othayoth
   email: ojayanth@in.ibm.com