Remove or suppress crawling for the OPAM 1.1 and 1.2 manuals

[dra27]

cf. https://discuss.ocaml.org/t/opam-installer-script-trying-to-install-a-very-old-opam-and-ocaml-compiler/14856/3

FWIW, I think that 6 years after the release of opam 2.0 we can probably just remove the 1.1 and 1.2 manuals from the site, but we should otherwise configure robots.txt to suppress them being indexed.