julia: remove 0.6.4 (julia_06)

[markuskowa]

Motivation for this change

Remove Julia 0.6, which is not supported anymore (https://julialang.org/downloads/oldreleases.html)
and fails on Hydra since the latest update of arpack to 3.6.3.

Partially resolves #51506
Needs backport to 19.03 (#56826)

CC @7c6f434c

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[markuskowa]

Backported to 19.03 in 7a32dfe