opam 2.2 misdiagnosing parse errors for opam-version: "2.2"

[dra27]

Slightly academic, as there were no changes for 2.2, but this affects #5665.

For

opam-version: "2.1"
GARBAGE

opam lint gives a parse error, which is correct.

For

opam-version: "2.2"
GARBAGE

it gives "unsupported or missing file format version; should be 2.0 or older", which is incorrect (it should still be a path error). The issue is failing to handling opam-file-format 2.1.x signalling a parse error for a future opam syntax.