Upgrade to cmdliner 2.0.0

[kit-ty-kate]

Fixes #6425

Issues to address:

• I'm personally unconvinced about the CMDLINER_LEGACY_PREFIXES=true escape hatch. I would much rather have a global reference (e.g. kit-ty-kate/cmdliner@94ecc8a) or whatever to make sure the behaviour doesn't change between two versions while we weren't looking
• The escape hatch in combination with ?env doesn't work with Arg.enum. Using the example patch above, this issue would be fixed.

If we decide to fork cmdliner with the following patch above master kit-ty-kate/cmdliner@1565763, those issues wouldn't be there anymore at the cost of extra maintenance.

cc @dbuenzli

[dbuenzli]

Note that the idea of #6425 was to simply suppress unabstract uses of converter and make it compile with 1.3.0 without deprecation warnings to allow for a smooth eco-system transition. This uses 2.0.0 APIs.

Regarding the introduction of a global reference the answer is still no: global references are not compositional. The last thing I want to see is libraries fighting to set that value or change it behind user's back.