Replace json.SkipFunc with errors.ErrUnsupported (Go 1.27 json/v2 API).

encoding/json/v2 graduated from GOEXPERIMENT to a default-on package in Go
1.27, and the API review dropped the json.SkipFunc sentinel.  The mechanism it
named still exists, but the signal a type-specific (un)marshal function returns
to decline a value and fall through to the next applicable function is now any
error matching errors.ErrUnsupported -- see the JoinMarshalers/JoinUnmarshalers
documentation and arshal_funcs.go:147 in the Go 1.27 sources.

Both call sites here decline before touching the decoder (PeekKind and
UnreadBuffer read no tokens), which is what json/v2 requires: it only forwards
ErrUnsupported as a skip when the decoder depth and length are unchanged,
otherwise it rejects the call as an unsupported mutation.

Without this the package fails to compile against Go 1.27 with
"undefined: json.SkipFunc".

--- a/pkg/x/json/json.go
+++ b/pkg/x/json/json.go
@@ -4,6 +4,7 @@
 	"bytes"
 	"encoding/json/jsontext"
 	"encoding/json/v2"
+	"errors"
 	"io"
 
 	"golang.org/x/xerrors"
@@ -106,7 +107,7 @@
 
 		// Check visited set to avoid infinity loops
 		if visited.Contains(start) {
-			return json.SkipFunc
+			return errors.ErrUnsupported
 		}
 		visited.Append(start)
 
--- a/pkg/iac/scanners/cloudformation/parser/parameter.go
+++ b/pkg/iac/scanners/cloudformation/parser/parameter.go
@@ -54,7 +54,7 @@
 		}
 		return nil
 	}
-	return json.SkipFunc
+	return errors.ErrUnsupported
 }
 
 func (p *Parameter) Type() cftypes.CfType {