chore: replace slices.Contains with utils.Contains

We were getting the following gosec error when using `slices.Contains`:

```
could not import slices (invalid package name: "")
```

This is because:

- `slices` was added in Go 1.21
- Support for Go 1.21 was added in gosec 2.17
- We are currently using gosec 2.15.0 and via salus 3.2.5 via federacy/scan-action which pulls `coinbase/salus:latest`

Salus 3.2.6 has been released with the gosec version bump but it isn't tagged `latest`. I've filed coinbase/salus#880 to address.

For now, the easiest thing to do was just to not use the `slices` module, which was easy because we already have a util which does the same thing.

pkg/configuration/flags.go

@@ -16,9 +16,8 @@ limitations under the License.
 package configuration
 
 import (
-	"slices"
-
 	"github.com/DopplerHQ/cli/pkg/models"
+	"github.com/DopplerHQ/cli/pkg/utils"
 )
 
 func GetFlag(flag string) bool {
@@ -71,7 +70,7 @@ func GetFlagDefault(flag string) bool {
 
 func IsValidFlag(flag string) bool {
 	flags := models.GetFlags()
-	return slices.Contains(flags, flag)
+	return utils.Contains(flags, flag)
 }
 
 func IsAnalyticsEnabled() bool {