Prevent stdlib compat entries from breaking old Julia versions

[DilumAluthge]

In old Julia versions, Pkg.test sets the version numbers of all stdlibs to 0.0.0. So, if we don't allow 0.0.0, then Pkg.test will throw a resolver error.

Example of this breaking: https://github.com/JuliaRegistries/RegistryCI.jl/pull/557 (e.g. this CI log).

See also:

1. https://github.com/JuliaBinaryWrappers/MozillaCACerts_jll.jl/blob/main/Project.toml
2. https://github.com/JuliaRegistries/General/blob/master/jll/M/MozillaCACerts_jll/Compat.toml
3. https://github.com/JuliaRegistries/General/blob/master/jll/M/MozillaCACerts_jll/Versions.toml