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: I56c3813cd3f5feb5f0b614786dea374efe19dc79
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/OWNERS b/OWNERS
index 8797e73..345f075 100644
--- a/OWNERS
+++ b/OWNERS
@@ -32,7 +32,7 @@
 # data within them will be kept sorted.
 
 owners:
-- gmills@linux.vnet.ibm.com
+- gunnar@gmills.xyz
 - kirankumarb@ami.com
 
 reviewers:
@@ -42,7 +42,7 @@
 
 openbmc:
 - name: Gunnar Mills
-  email: gmills@linux.vnet.ibm.com
+  email: gunnar@gmills.xyz
   discord: GunnarM
 - name: Kirankumar Ballapalli
   email: kirankumarb@ami.com