Consider moving graph cache from .graph to .cache/graph to match XDG specification

[JustinGrote]

Powershell 7 follows the XDG spec for user folder data, this module should as well to help discovery and filesystem linking.

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

$XDG_CACHE_HOME defines the base directory relative to which user-specific non-essential data files should be stored. If $XDG_CACHE_HOME is either not set or empty, a default equal to $HOME/.cache should be used. AB#10167

https://docs.microsoft.com/en-us/powershell/scripting/install/installing-powershell-core-on-linux?view=powershell-7.1#paths

[sebastienlevert]

@calebkiage @timayabi2020 this seems like a best practice we should be following. I wouldn't prioritize it as of yet, but I agree with @JustinGrote that this would provide a better unified experience for UNIX PowerShell devs.

Now, will this be considered a breaking change (or at least, breaking cached tokens for users using it, forcing new authentication)?