replace str downcase for str lowercase

[AucaCoyan]

nu 114 raises:

Warning: nu::parser::deprecated

  ⚠ Command deprecated.
     ╭─[/home/aucac/other-repos/nu/nu_scripts/modules/fnm/fnm.nu:105:35]
 104 │                     let answer = (input "Install it? [y/N] ")
 105 │                     if ($answer | str downcase) == "y" {
     ·                                   ──────┬─────
     ·                                         ╰── str downcase was deprecated in 0.114.0 and will be removed in a future release.
 106 │                         let install_res = (do { ^fnm install ...$fnm_config.install_flags } | complete)
     ╰────
  help: Use `str lowercase` instead.

So this PR replaces all str downcase for str lowercase

[fdncred]

Thanks