.gitreview: Update default branch from master to main

The master is deprecated in favor of the main.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: I58001819079bc880e8cde1c3a6756ff6c8a1c016
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79743
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

.gitreview

@@ -2,4 +2,4 @@
 host=review.coreboot.org
 port=29418
 project=coreboot
-defaultbranch=master
+defaultbranch=main