Use `AbstractAlgebra.should_show_banner`

[fingolfin]

... instead of

   show_banner = allowbanner && isinteractive_manual && isinteractive() &&
                !any(x->x.name in ["Oscar"], keys(Base.package_locks)) &&
                get(ENV, "SINGULAR_PRINT_BANNER", "true") != "false"

etc.

Similar to what https://github.com/Nemocas/Nemo.jl/pull/1714 by @lgoettgens did