ci: move iOS13-Swift under xcodegen mgmt

[armcknight]

following #5116, moving app projects to xcodegen. this is the last app that was under iOS-Swift.xcodeproj, so that xcodeproj file is now able to be removed here.

Also, renamed the directory containing the ui test sources from iOS13-SwiftTests to iOS13-Swift-UITests for consistency and clarity.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.688%. Comparing base (1707309) to head (0cc9ac3).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           armcknight/ci/ios-swift6-xcodegen     #5117       +/-   ##
=======================================================================
- Coverage                             92.773%   92.688%   -0.086%     
=======================================================================
  Files                                    677       677               
  Lines                                  84273     84273               
  Branches                               30659     29584     -1075     
=======================================================================
- Hits                                   78183     78111       -72     
- Misses                                  5987      6065       +78     
+ Partials                                 103        97        -6     

see 18 files with indirect coverage changes

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Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1237.83 ms	1259.30 ms	21.47 ms
Size	22.31 KiB	851.57 KiB	829.26 KiB