Update Jon Johnson's email

Signed-off-by: Jon Johnson <jonjohnsonjr@gmail.com>

MAINTAINERS

@@ -1,7 +1,7 @@
 Brandon Mitchell <git@bmitch.net> (@sudo-bmitch)
 Derek McGowan <derek@mcg.dev> (@dmcgowan)
 Jimmy Zelinskie <jimmy@zelinskie.com> (@jzelinskie)
-Jon Johnson <jonjohnson@google.com> (@jonjohnsonjr)
+Jon Johnson <jon.johnson@chainguard.dev> (@jonjohnsonjr)
 Josh Dolitsky <josh@bloodorange.io> (@jdolitsky)
 Mike Brown <brownwm@us.ibm.com> (@mikebrow)
 Stephen Day <stevvooe@gmail.com> (@stevvooe)