Update pandoc-ast to 1.21

[FredeEB]

The ast has been updated to 1.21, which means the assertion mentioned in #5 is failing 👍

[lrosenthol]

found the same thing - and happy to help with PR

[oli-obk]

Yes, a PR would be great. We should probably change the assertion to handling all versions up to 1.21

[sbarral]

Declaring support for 1.21 may have been a bit premature, it looks like the table serialization format has changed.

This simple table:

| A | B |  
|---|---|
| a | b |

works fine with pandoc 2.9.2 but triggers the following Serde error with pandoc 2.10.1:

thread 'main' panicked at 'json is not in the pandoc format: Error("invalid type: string \"\", expected adjacently tagged enum Inline", line: 231, column: 5)

This is presumably related to pandoc #1024.