perhaps switch validator so as to support newest versions of java?

Nobody has claimed this yet.

I have found that the [current](https://github.com/matteosecli/pdf2archive/tree/master/verapdf) `verapdf` validator errors on newer versions of `java`:
```
$ java -version
openjdk version "15.0.2" 2021-01-19
OpenJDK Runtime Environment (build 15.0.2+7)
OpenJDK 64-Bit Server VM (build 15.0.2+7, mixed mode)
```
and then
```
$ ./pdf2archive --validate ~/Downloads/boo.pdf
=== Welcome to PDF2ARCHIVE ===
GPL Ghostscript 9.54.0: Unrecoverable error, exit code 1
Creating the definition file...
Compressing PDF & embedding fonts...
Converting to PDF/A-1B...
Removing temporary files...
Done, now ESSE3 is happy! ;)
Validating...
./verapdf/verapdf: line 125: [: 150 2021-01-19: integer expression expected
Exception in thread "main" java.lang.NoClassDefFoundError: javax/xml/bind/JAXBException
at org.verapdf.apps.Applications.createConfigManager(Applications.java:75)
at org.verapdf.apps.Applications.createAppConfigManager(Applications.java:87)
at org.verapdf.cli.VeraPdfCli.(VeraPdfCli.java:46)
at org.verapdf.apps.GreenfieldCliWrapper.main(GreenfieldCliWrapper.java:34)
Caused by: java.lang.ClassNotFoundException: javax.xml.bind.JAXBException
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:606)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:168)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
... 4 more
```
On the other hand, I've found [this](https://pdfbox.apache.org/) `java` pdf library, that comes with a convenient [command-line validator](https://www.apache.org/dyn/closer.lua/pdfbox/3.0.0-RC1/preflight-app-3.0.0-RC1.jar) (a `jar` file).

I have checked that it agrees with your current `vera` validator on a number of files, both pre- and post-conversion. Before:
```
$ java -jar ~/Downloads/preflight-app-3.0.0-RC1.jar ~/Downloads/boo.pdf
---
The file boo.pdf is not a valid PDF/A-1b file, error(s) :
1.2.1 : Body Syntax error, EOL expected before the 'endobj' keyword at offset 471948
1.2.1 : Body Syntax error, EOL expected before the 'endobj' keyword at offset 475853
1.2.1 : Body Syntax error, EOL expected before the 'endobj' keyword at offset 468071
...
```
After:
```
$ java -jar ~/Downloads/preflight-app-3.0.0-RC1.jar ~/Downloads/boo-PDFA.pdf
---
The file boo-PDFA.pdf is a valid PDF/A-1b file
```
This works on both
```
$ java -version
---
openjdk version "15.0.2" 2021-01-19
OpenJDK Runtime Environment (build 15.0.2+7)
OpenJDK 64-Bit Server VM (build 15.0.2+7, mixed mode)
```
and
```
$ java -version
---
openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-8u292-b10-0ubuntu1~20.04-b10)
OpenJDK 64-Bit Server VM (build 25.292-b10, mixed mode)
```
(the two versions I have handy right now).

Start with the verapdf/verapdf wrapper, especially line 125, and run ./pdf2archive --validate with the Java 15 version shown in the report. Check the validator invocation and its compatibility with newer Java, then confirm validation works for converted and unconverted PDFs as in the examples while remaining consistent with the current validator.

Written by the indexing model from the issue text.