openssl_1_0_2: mark as insecure; fixes #77503 (kinda)

No vulnerabilities are know so far (to me), but still I'd go this way.
Especially for 20.03 it seems better to deprecate it before official
release happens.

Current casualties:
$ ./maintainers/scripts/rebuild-amount.sh --print HEAD HEAD^
Estimating rebuild amount by counting changed Hydra jobs.
     87 x86_64-darwin
    161 x86_64-linux