Allow MKL_jll 2021

[mcabbott]

This widens the version bounds on MKL_jll.

But I mark it as a draft for now, because when testing on 1.6, this happens:

     Testing Running tests...
ERROR: LoadError: InitError: Artifact "MKL" is a lazy artifact; package developers must call `using LazyArtifacts` in MKL_jll before using lazy artifacts.
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:33
  [2] _artifact_str(__module__::Module, artifacts_toml::String, name::SubString{String}, path_tail::String, artifact_dict::Dict{String, Any}, hash::Base.SHA1, platform::Base.BinaryPlatforms.Platform, lazyartifacts::Any)
    @ Artifacts ~/.julia/dev/julia/usr/share/julia/stdlib/v1.7/Artifacts/src/Artifacts.jl:547
  [3] invokelatest(::Any, ::Any, ::Vararg{Any}; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Base ./essentials.jl:714
  [4] invokelatest(::Any, ::Any, ::Vararg{Any})
    @ Base ./essentials.jl:712
  [5] macro expansion
    @ ~/.julia/dev/julia/usr/share/julia/stdlib/v1.7/Artifacts/src/Artifacts.jl:674 [inlined]
  [6] find_artifact_dir()
    @ MKL_jll ~/.julia/packages/JLLWrappers/KuIwt/src/wrapper_generators.jl:15
  [7] __init__()
    @ MKL_jll ~/.julia/packages/MKL_jll/SJiaS/src/wrappers/x86_64-apple-darwin.jl:9
  [8] _include_from_serialized(path::String, depmods::Vector{Any})
    @ Base ./loading.jl:670
  [9] _require_search_from_serialized(pkg::Base.PkgId, sourcepath::String)

I'm not too sure whether this means changes are needed here or there.

Tests are fine on Julia 1.5, and on 1.3 says CI.