feat: help user with lockfile update error

[ruben-arts]

fixes: #2679

Message look like this:

❯ pixi i
  × Failed to load lock file from `/Users/rubenarts/dev/pixi/pixi.lock`
  ╰─▶ The lock file version is 7, but only up to including version 6 is supported by the current version.
  help: Please update pixi to the latest version and try again.

[nichmor]

tested manually by updating our pixi.lokc version - works for me!