fix(linux)!: use XDG spec for fetching Linux configuration directory

[rafaelsgirao]

Description

Makes Sunshine follow the XDG Base Directory specification when setting the configuration directory under Linux:

$XDG_CONFIG_HOME defines the base directory relative to which user-specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

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

Additionally, following this change $CONFIGURATION_DIRECTORY will also be checked and used, if set. This is used by systemd services that explicitly specify a ConfigurationDirectory= option:
https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#RuntimeDirectory=

This should have minimal to no impact on existing Sunshine instances, since users with $XDG_CONFIG_HOME set will typically have their Sunshine instance still look for files in $HOME/.config .
Existing Sunshine instances running under e.g a hand-made systemd service may have an unexpected configuration directory change, if they're using ConfigurationDirectory= , but I think this is a very niche use-case. Still labeling as a breaking change, just in case.

Screenshot

N/A

Issues Fixed or Closed

N/A

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
  I thought of putting a link to the spec above my changes, but I didn't see anything like that elsewhere in the code.
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components
  N/A (?)

Branch Updates

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must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

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