Add version validation to install commands

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/version_utils.go

@@ -18,6 +18,7 @@ package main
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/fluxcd/flux2/internal/utils"
 	"github.com/fluxcd/flux2/pkg/manifestgen/install"
@@ -28,6 +29,10 @@ func getVersion(input string) (string, error) {
 		return rootArgs.defaults.Version, nil
 	}
 
+	if input != install.MakeDefaultOptions().Version && !strings.HasPrefix(input, "v") {
+		return "", fmt.Errorf("targeted version '%s' must be prefixed with 'v'", input)
+	}
+
 	if isEmbeddedVersion(input) {
 		return input, nil
 	}