gcc7, gcc7Stdenv, gfortran7: remove old implementation

[trofi]

Description of changes

gcc-7.5.0 was released in Nov 14, 2019, 5 years ago. It's a branch that
went out of support years ago. Numerous bugs never get backported to
this version.

Let's remove it.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[trofi]

Result of nixpkgs-review pr 342242 run on x86_64-linux 1

1 package built:

• blackmagic-desktop-video

[trofi]

If #342235 is merged earlier I can rebase past it to get 0 rebuilds on this PR.

[trofi]

Will need cuda changes:

1|   gccMajorVersion = nvccCompatibilities.${cudaVersion}.gccMaxMajorVersion;
           12|   cudaStdenv = stdenvAdapters.useLibsFrom stdenv pkgs."gcc${gccMajorVersion}Stdenv";
             |                                                  ^
           13|   passthruExtra = {
       Did you mean one of gcc8Stdenv, gcc9Stdenv, gccStdenv, gcc10Stdenv or gcc11Stdenv?

[emilazy]

It needs to wait until after branch‐off; see #342112.

I intend to remove GCCs 7 through 10 at that point to align with upstream’s support policy. (I have confirmed that all packages using GCC 9 should be possible to migrate off, and nothing uses 8. It’s possible 10 will have to stick around for now, depending on how that goes.)

[emilazy]

Also QMK would need looking at, I think?

[emilazy]

(Thank you for taking the charge in getting these cleaned up!)