Revert "go.mod/spf13: Bump versions"

This reverts commit fc2fc9a4f4.
go1.14 fails `make build*` command.
Go version should be updated.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

go.mod

@@ -18,9 +18,9 @@ require (
 	github.com/panjf2000/ants/v2 v2.4.0
 	github.com/paulmach/orb v0.2.2
 	github.com/prometheus/client_golang v1.11.0
-	github.com/spf13/cast v1.3.1
-	github.com/spf13/cobra v1.1.3
-	github.com/spf13/viper v1.8.0
+	github.com/spf13/cast v1.3.0
+	github.com/spf13/cobra v1.0.0
+	github.com/spf13/viper v1.7.0
 	github.com/stretchr/testify v1.7.0
 	go.etcd.io/bbolt v1.3.5
 	go.uber.org/atomic v1.8.0