Bump Minimum Swift Version to 5.9

[cpisciotta]

Bumping to Swift 5.9 introduces some particularly helpful new features, namely the package access modifier (SE-386). Bumping to Swift 5.9 is a breaking change for consumers of XcbeautifyLib, such as Tuist, since their packages will also require Swift 5.9 too. It is not a breaking change if a consumer simply uses the xcbeautify CLI. The package access modifier will be particularly helpful to introduce new logic into XcbeautifyLib that the executable, xcbeautify, needs, without unnecessarily making the additions public to third-party consumers.

[cpisciotta]

Closing, since this'll drop SPM support for 5.7 and 5.8.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (083aedf) 85.83% compared to head (a6725cd) 85.88%.

Files	Patch %	Lines
...ces/XcbeautifyLib/Renderers/TerminalRenderer.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
+ Coverage   85.83%   85.88%   +0.05%     
==========================================
  Files          13       13              
  Lines        1842     1842              
==========================================
+ Hits         1581     1582       +1     
+ Misses        261      260       -1     

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