Use slices from stdlib over golang.org/x/exp/slices

[oliversun9]

Using golang.org/x/exp/slices might cause problems like

../protovalidate/tools/protovalidate-conformance/internal/results/result.go:226:30: type func(a *validate.Violation, b *validate.Violation) bool of func(a, b *validate.Violation) bool {…} does not match inferred type func(a *validate.Violation, b *validate.Violation) int for func(a E, b E) int

especially when you are in a Go workspace.