refactor: drop support for Swift 5.2

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Swift 5.2 and Xcode 11 isn't supported by the AppStore anymore per Apple:

iPhone and iPad apps. Starting April 26, 2021, all iPhone and iPad apps submitted to the App Store must be built with Xcode 12 and the iOS 14 SDK or later. The iOS SDK provides access to exciting new features like App Clips, Home screen widgets, ARKit, RealityKit, and much more.

Dropping support for 5.2 allows the SDK to be designed with the latest advancements of 5.3+; particularly:

• availability of self (the SDK is currently designed to use self a specific way because of 5.2)
• key paths (the SDK is currently designed to not use keyPath's as functions because of 5.2)
• much more

SwiftPackageIndex has also stopped building Swift 5.2 and only builds Swift 5.3+: https://swiftpackageindex.com/parse-community/Parse-Swift/builds

Related issue: #n/a

Approach

Remove Swift 5.2 from CI along with any related compiler checks.

TODOs before merging

• Add entry to changelog

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #356 (b421ebf) into main (2cb662a) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #356      +/-   ##
==========================================
- Coverage   88.00%   87.90%   -0.11%     
==========================================
  Files         114      114              
  Lines       12209    12198      -11     
==========================================
- Hits        10745    10723      -22     
- Misses       1464     1475      +11     

Impacted Files	Coverage Δ
Sources/ParseSwift/Extensions/Data.swift	58.33% <ø> (-41.67%)	⬇️
Sources/ParseSwift/Objects/ParseUser+async.swift	100.00% <ø> (ø)
Sources/ParseSwift/Objects/ParseUser+combine.swift	95.55% <ø> (ø)
Sources/ParseSwift/Objects/ParseUser.swift	86.20% <ø> (ø)
Sources/ParseSwift/Coding/ParseEncoder.swift	77.31% <0.00%> (-0.46%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	85.19% <0.00%> (+0.45%)	⬆️

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[cbaker6]

@mtrezza can you remove spm-test-5_2 and xcode-test-earliest as a requirement for the CI?