Bump MACOSX_DEPLOYMENT_TARGET to 10.14.6 (#950)

[TP-113]

Description

Bump MACOSX_DEPLOYMENT_TARGET in Runner.xcodeproj to 10.14.6.
Change Debug Information Format / Debug from DWARF to DWARF with dSYM File due to fix the error below.

warning: Run script build phase '[firebase_crashlytics] Crashlytics Upload Symbols' will be run during every build because it does not specify any outputs. To address this warning, either add output dependencies to the script phase, or configure it to run in every build by unchecking "Based on dependency analysis" in the script phase. (in target 'Runner' from project 'Runner')
src/flutter_app/gallery/macos/Pods/Pods.xcodeproj: warning: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.11, but the range of supported deployment target versions is 10.13 to 13.3.99. (in target 'nanopb' from project 'Pods')
src/flutter_app/gallery/macos/Pods/Pods.xcodeproj: warning: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.11, but the range of supported deployment target versions is 10.13 to 13.3.99. (in target 'PromisesSwift' from project 'Pods')
src/flutter_app/gallery/macos/Pods/Pods.xcodeproj: warning: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.11, but the range of supported deployment target versions is 10.13 to 13.3.99. (in target 'PromisesObjC' from project 'Pods')
src/flutter_app/gallery/macos/Pods/Pods.xcodeproj: warning: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.12, but the range of supported deployment target versions is 10.13 to 13.3.99. (in target 'GoogleUtilities' from project 'Pods')
src/flutter_app/gallery/macos/Pods/Pods.xcodeproj: warning: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 10.12, but the range of supported deployment target versions is 10.13 to 13.3.99. (in target 'GoogleDataTransport' from project 'Pods')
warning: Run script build phase 'Run Script' will be run during every build because it does not specify any outputs. To address this warning, either add output dependencies to the script phase, or configure it to run in every build by unchecking "Based on dependency analysis" in the script phase. (in target 'Flutter Assemble' from project 'Runner')

Tests

I made sure flutter run -d macos was successful.

Issues

Fixes #950

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[guidezpl]

Please revert the changes in this file

[TP-113]

@guidezpl Thank you for your review.
I reverted the change in "pubspec.lock".

[guidezpl]

Thanks!