feat(ios) bump minimum required iOS version to 15.1

RN 0.76 will be doing this change, so let's get ahead.

THis puts the iPhione 6S as the baseline model, which was released in
September 2015.

ios/Podfile

     {paths: [process.argv[1]]},
   )', __dir__]).strip
 
-platform :ios, '13.4'
+platform :ios, '15.1'
 workspace 'jitsi-meet'
 
 install! 'cocoapods', :deterministic_uuids => false
     end
     target.build_configurations.each do |config|
       config.build_settings['SUPPORTS_MACCATALYST'] = 'NO'
-      config.build_settings['IPHONEOS_DEPLOYMENT_TARGET'] = '13.4'
+      config.build_settings['IPHONEOS_DEPLOYMENT_TARGET'] = '15.1'
       config.build_settings['OTHER_SWIFT_FLAGS'] = '$(inherited) -no-verify-emitted-module-interface'
     end
   end

ios/Podfile.lock

   SocketRocket: f32cd54efbe0f095c4d7594881e52619cfe80b17
   Yoga: e5b887426cee15d2a326bdd34afc0282fc0486ad
 
-PODFILE CHECKSUM: 513caf6e662086d8bae95a064c18423f0c07fa01
+PODFILE CHECKSUM: 79119d2af4f01a2584749dc3926902e66572d896
 
 COCOAPODS: 1.15.2

ios/app/app.xcodeproj/project.pbxproj

 					"$(inherited)",
 					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 				);
-				IPHONEOS_DEPLOYMENT_TARGET = 13.4;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.1;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				ONLY_ACTIVE_ARCH = YES;
 				OTHER_CFLAGS = "$(inherited)";
 					"$(inherited)",
 					/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include,
 				);
-				IPHONEOS_DEPLOYMENT_TARGET = 13.4;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.1;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				OTHER_CFLAGS = "$(inherited)";
 				OTHER_CPLUSPLUSFLAGS = "$(inherited)";

ios/sdk/sdk.xcodeproj/project.pbxproj

 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 13.4;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.1;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				ONLY_ACTIVE_ARCH = YES;
 				OTHER_CFLAGS = "$(inherited)";
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 13.4;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.1;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				OTHER_CFLAGS = "$(inherited)";
 				OTHER_CPLUSPLUSFLAGS = "$(inherited)";