Improve Pandoc version incompatibility error messages

[max-heller]

pandoc-ast/src/lib.rs

Lines 36 to 50 in e99d773

	// test pandoc version
	if let Some((major, minor)) = pandoc_version(obj) {
	if !(major == 1 && minor >= 20) {
	panic!(
	"Pandoc version mismatch: \
	`pandoc-ast` expects pandoc version 1.20 or newer, got {}.{}",
	major, minor
	);
	}
	} else {
	panic!(
	"Unable to parse pandoc version from JSON. \
	Please update your pandoc to at least version 1.18 or use an older version of `pandoc-ast`"
	);
	}

The first message says that 1.20 is required, while the second says 1.18. I'm assuming 1.20 is intended since that's what the code checks.

Also, the version this is checking is Pandoc's AST version (the version of the pandoc-types package), which is versioned separately from pandoc. This clarifies that and states the corresponding minimum Pandoc version (2.8 assuming the 1.20 AST version requirement is the correct one).