PDF/A-4 (PDF 2.0)

PDF/A-4 was released in 2020 based on PDF 2.0, but apparently it is still not possible to select this standard in the [PdfRendererBuilder#PdfAConformance](https://github.com/danfickle/openhtmltopdf/blob/open-dev-v1/openhtmltopdf-pdfbox/src/main/java/com/openhtmltopdf/pdfboxout/PdfRendererBuilder.java) enum. This feature request is probably not solved simply by adding this value to the enum and depends heavily on PDFBox. I don't know, but is there a plan when openhtmltopdf will support this standard?

```
/**
* Various level of PDF/A conformance:
*
* PDF/A-1, PDF/A-2 and PDF/A-3
*/
public enum PdfAConformance {
NONE(-1, "", 0f),
PDFA_1_A(1, "A", 1.4f), PDFA_1_B(1, "B", 1.4f),
PDFA_2_A(2, "A", 1.7f), PDFA_2_B(2, "B", 1.7f), PDFA_2_U(2, "U", 1.7f),
PDFA_3_A(3, "A", 1.7f), PDFA_3_B(3, "B", 1.7f), PDFA_3_U(3, "U", 1.7f);

[...]
}
```