feat(deps): reverting OPA v0.29.4 to v0.28.0

Reverting PR #3503

These are the two queries failing:
https://github.com/Checkmarx/kics/blob/master/assets/queries/k8s/volume_mount_with_os_directory_write_permissions/query.rego
https://github.com/Checkmarx/kics/blob/master/assets/queries/terraform/kubernetes/volume_mount_with_os_directory_write_permissions/query.rego

We should check if this has anything to do with either:
open-policy-agent/opa#3501
open-policy-agent/opa#3505

*UPDATE*
This new query #3651 is failing after the update.
This might be related to this issue open-policy-agent/opa#3546

Signed-off-by: Rogério Peixoto <rogerio.peixoto@checkmarx.com>

go.mod

@@ -16,7 +16,7 @@ require (
 	github.com/mailru/easyjson v0.7.7
 	github.com/mitchellh/go-wordwrap v1.0.1 // indirect
 	github.com/moby/buildkit v0.8.3
-	github.com/open-policy-agent/opa v0.29.4
+	github.com/open-policy-agent/opa v0.28.0
 	github.com/pkg/errors v0.9.1
 	github.com/rs/zerolog v1.23.0
 	github.com/spf13/cobra v1.1.3