Pull request 2184: 6851 upstream mode reset

Updates #6851.

Squashed commit of the following:

commit ffc50daff8e9a2b0fd48f10c5e66cdc0ab350488
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Mar 25 17:55:39 2024 +0300

    all: fix changelog

commit cdc2193875e72f6504cf283c84e1e6c6768d4f6e
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Mar 25 17:28:06 2024 +0300

    dnsforward: fix upstream mode set

internal/dnsforward/http_test.go

@@ -29,6 +29,10 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// TODO(e.burkov):  Use the better approach to testdata with a separate
+// directory for each test, and a separate file for each subtest.  See the
+// [configmigrate] package.
+
 // emptySysResolvers is an empty [SystemResolvers] implementation that always
 // returns nil.
 type emptySysResolvers struct{}