It's a bit of a hassle to manually manage the supported algorithms,
which are easy to get wrong. These supported algorithms are used by the
client, while the server is more conservative and mostly uses Go's
default algorithms. Let's just filter the values we know are
problematic until Go ships something for us.

This effectively adds back a number of host key algorithms, such as
KeyAlgoED25519 (not to be confused with CertAlgoED25519). But since only
the client (e.g. fleeting plugin) only uses `fips.SupportedAlgorithms()`
at the moment, changing the supported host keys doesn't affect the
client. It will become important if gitlab-sshd ever switches to
`fips.SupportedAlgorithms()` from `fips.DefaultAlgorithms()`.

Given renovate is our defacto tool for managing dependency
bumps these days, this script and the corresponding docs are
likely outdated.

chore: removes downsteram-vendor.sh file and corresponding docs

See merge request !220



Merged-by: Igor <iwiedler@gitlab.com>
Approved-by: Igor <iwiedler@gitlab.com>
Co-authored-by: e_forbes <eforbes@gitlab.com>

fix: simplify implementation of FIPS SupportedAlgorithms()

See merge request !218



Merged-by: Stan Hu <stanhu@gmail.com>
Approved-by: João Pereira <jpereira@gitlab.com>