ardour: add ardour 6 and make it the default

[magnetophon]

We leave Ardour 5 in, since Ardour 6 has a new latency compensation
algorithm, so your mixes might end up sounding different in the new version.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[x42]

looks like you've missed libpulse-dev and libwebsockes-dev packages as dependency

[magnetophon]

looks like you've missed libpulse-dev and libwebsockes-dev packages as dependency

@x42
pulseaudio is in there,and it claims to build with it. I don't have pulse installed,so have not tested.

seems that in order to build with libwebsockets, you also need libuv (assuming that's what provides uv.h), but it only fails halfway trough the build.
Both are in now, thanks.

[prusnak]

I made an earlier PR here: #87274 - Please make sure you cherry-pick all of the changes from my PR that you want to include. I will close my PR in favor of this one afterward.

Also it's always a good idea to check for other PRs, before starting your own, to avoid duplicate work.

[magnetophon]

@prusnak Oh,I thought I had checked for other PRs, sorry!
I incorporated most of your changes, but yours has the same issue as @veprbl 's: it replaces ardour5 when installing 6, which what I wanted to avoid, see the commit message about the new latency compensation.
What do you suggest?

[veprbl]

@magnetophon Forgot to push the updated commits?

[magnetophon]

@veprbl Ah, yes, something went wrong.

[veprbl]

LGTM. Just a few minor comments.

[magnetophon]

@veprbl Thanks, all done.

[magnetophon]

Something went wrong here, I'm trying to fix it!