fix STDLIBS_BY_VERSION on non-latest version of Julia

[KristofferC]

Should fix the test error in #2425

Edit: This still need some tweaks

[IanButterworth]

If it helps, there are some debug prints in the tests on master that are now backported in #2425
Over there they show

┌ Error: STDLIBS_BY_VERSION out of date!  Manually fix given the info below, or re-run generate_historical_stdlibs.jl!
└ @ Main.PkgTests.NewTests ~/work/Pkg.jl/Pkg.jl/test/new.jl:2524
length((Pkg.Types.STDLIBS_BY_VERSION[end])[2]) = 58
length(Pkg.Types.load_stdlib()) = 57
setdiff((Pkg.Types.STDLIBS_BY_VERSION[end])[2], Pkg.Types.load_stdlib()) = Pair{Base.UUID, String}[UUID("47c5dbc3-30ba-59ef-96a6-123e260183d9") => "LLVMLibUnwind_jll", UUID("8e850b90-86db-534c-a0d3-1478176c7d93") => "libblastrampoline_jll"]
setdiff(Pkg.Types.load_stdlib(), (Pkg.Types.STDLIBS_BY_VERSION[end])[2]) = Pair{Base.UUID, String}[UUID("a83860b7-747b-57cf-bf1f-3e79990d037f") => "LibOSXUnwind_jll"]
STDLIBS_BY_VERSION up-to-date: Test Failed at /home/runner/work/Pkg.jl/Pkg.jl/test/new.jl:2529

[fredrikekre]

Looks like this is already on release-1.6