Follow App Store submit rule, upgrade the minimum Xcode version to Xcode 10.0

[dreampiggy]

New Pull Request Checklist

• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• I have added the required tests to prove the fix/feature I am adding

• I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

This merge request fixes / refers to the following issues: #2372 #2766

Pull Request Description

See the similar PR #2372

Xcode 11 is now available via WWDC 2019. And Xcode 9 is 2 years ago.

According to App Store Submit Rule. All the new or upgrade version for Apps MUST link iOS 12 SDK & Xcode 10. So it's time to drop Xcode 9.0 support. See: https://developer.apple.com/news/?id=03202019a

Starting March 27, 2019, all new apps and app updates for iPhone or iPad, including universal apps, must be built with the iOS 12.1 SDK or later and support iPhone XS Max or the 12.9-inch iPad Pro (3rd generation). Screenshots for these devices will also be required. All new apps and app updates for Apple Watch will need to be built with the watchOS 5.1 SDK or later and support Apple Watch Series 4.

Beside this, Xcode 10 bring the support like Using ARC Objective-C object in C struct filed, New Build System, Xcode 10 File List, some of them can be helpful for us. Upgrating the developer toolchain version is easier than the minimum deployment target version as well.

[codecov]

Codecov Report

Merging #2780 into master will increase coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2780      +/-   ##
==========================================
+ Coverage   83.57%   83.86%   +0.28%     
==========================================
  Files          57       57              
  Lines        6285     6285              
==========================================
+ Hits         5253     5271      +18     
+ Misses       1032     1014      -18

Flag	Coverage Δ
#ios	84.78% <ø> (+0.17%)	⬆️
#macos	82.81% <ø> (+0.16%)	⬆️

Impacted Files	Coverage Δ
SDWebImage/SDWebImageIndicator.m	74.21% <ø> (ø)	⬆️
SDWebImage/SDWebImageManager.m	87.4% <0%> (-0.77%)	⬇️
SDWebImage/SDAnimatedImageView.m	79.85% <0%> (+2.45%)	⬆️
SDWebImage/SDWebImageDownloaderOperation.m	91.3% <0%> (+3.34%)	⬆️

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