feat: enable async/await on older OS's

[cbaker6]

New Pull Request Checklist

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

Issue Description

Xcode 13.2 backports async/await for older OS's (iOS13, tvOS13, watchOS6, and macOS10_15). See more here

Requires Xcode 13.2 or above to use async/await. Not compatible with Xcode 13.0/1. Still works with Xcode 11/12. Developers who want to use this should point to this branch as it won't be merged until Xcode 13.2 is officially released.

Related issue: #n/a

Approach

Remove OS requirement on async/await methods.

TODOs before merging

• Remove OS restrictions
• Make all async/await methods @MainActor's
• Add multi-core building for improved build times. More info here
• Add ParseRelation static methods and tests
• Improve documentation for async/await and Combine methods

[parse-github-assistant]

Thanks for opening this pull request!

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

[codecov]

Codecov Report

Merging #278 (26445a7) into main (ae6de04) will increase coverage by 0.15%.
The diff coverage is 72.04%.

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
+ Coverage   82.80%   82.96%   +0.15%     
==========================================
  Files         104      104              
  Lines       10622    10582      -40     
==========================================
- Hits         8796     8779      -17     
+ Misses       1826     1803      -23     

Impacted Files	Coverage Δ
...cation/3rd Party/ParseApple/ParseApple+async.swift	100.00% <ø> (ø)
.../3rd Party/ParseFacebook/ParseFacebook+async.swift	100.00% <ø> (ø)
...tication/3rd Party/ParseLDAP/ParseLDAP+async.swift	100.00% <ø> (ø)
...on/3rd Party/ParseTwitter/ParseTwitter+async.swift	100.00% <ø> (ø)
...tication/3rd Party/ParseTwitter/ParseTwitter.swift	100.00% <ø> (ø)
...Authentication/Internal/ParseAnonymous+async.swift	100.00% <ø> (ø)
...tication/Protocols/ParseAuthentication+async.swift	85.71% <ø> (ø)
Sources/ParseSwift/Extensions/URLSession.swift	72.46% <0.00%> (ø)
...es/ParseSwift/LiveQuery/ParseLiveQuery+async.swift	72.72% <ø> (ø)
...s/ParseSwift/Objects/ParseInstallation+async.swift	100.00% <ø> (ø)
... and 30 more

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