Migrate Carthage dependencies to use XCFrameworks

[danielrbrowne]

This PR:

• Migrates the Carthage dependencies used when building from source to be integrated using XCFrameworks (i.e. carthage bootstrap --use-xcframeworks)
  • Also updates Nimble to latest version to bring in Xcode 12.x build fixes
• Updates README to update the minimum Xcode supported version to 12.0 as a result of these changes

[codecov-commenter]

Codecov Report

Merging #176 (92a8cfb) into master (45cdf3c) will increase coverage by 0.44%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   89.67%   90.11%   +0.44%     
==========================================
  Files          58       58              
  Lines        2972     2974       +2     
==========================================
+ Hits         2665     2680      +15     
+ Misses        307      294      -13     

Impacted Files	Coverage Δ
Sources/PushNotifications.swift	88.00% <0.00%> (+0.19%)	:arrow_up:
Sources/Services/ServerSyncProcessHandler.swift	82.60% <0.00%> (+1.97%)	:arrow_up:
Sources/Models/ServerSyncJob.swift	74.22% <0.00%> (+8.24%)	:arrow_up:

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