pgvecto-rs: mark as broken in pg12 and pg13
Upstream (accidentally) broke support for postgresql 12 and 13 on
v0.2.1 by changing the signature of the `from_datum` function[^1].
This went unnoticed since the release branch `0.2` did not have CI.
Furthermore, they are removing support for these versions of postgresql
on v0.3.0[^2].
[^1]: https://github.com/tensorchord/pgvecto.rs/commit/97e861d51d5a5a8c314a6657e02d445a1edcaa30
[^2]: https://github.com/tensorchord/pgvecto.rs/issues/343