chore: update podspec to ~> 8.21

[techouse]

Summary

I updated

s.dependency 'mParticle-Apple-SDK/mParticle', '~> 8.21'
s.platform = :ios, '9.0'

I also amended

s.author = { 'mParticle' => '[email protected]' }

Testing Plan

• Tested with a local build using

mparticle_flutter_sdk:
  git:
    url: https://github.com/techouse/mparticle-flutter-sdk.git
    ref: chore/update-podspec

[techouse]

@rmi22186 should it maybe just point to 8.x, i.e.

s.dependency 'mParticle-Apple-SDK/mParticle', '~> 8'

[techouse]

Looks like I'll need to update the example Podfile.

[techouse]

@rmi22186 I have updated the example/ios/Podfile to point to

pod 'mParticle-Apple-SDK', '~> 8.21'

even thought I don't believe pinning it there is needed.