macos: Bump deployment target from 10.14 to 10.15

Fixes zulip#1116.

Done by changing the line in the Podfile, and following the
(recently added) comment there by changing a setting in the Xcode
GUI.

The Podfile change seems to be the only thing missing when Greg
attempted this bump:

(from zulip#1116)
> Naturally we're happy to bump up our minimum macOS deployment
> target — 10.15 is from 2019, and even increasing to last year's
> macOS 14 would be perfectly fine. But I spent a few minutes just
> now attempting to do so — editing the `MACOSX_DEPLOYMENT_TARGET =
> 10.14;` lines in the `project.pbxproj` file so they say 10.15 (or
> 13.0) instead, then also tried setting it in Xcode which added
> some new lines of that form — and the error persisted. (Perhaps
> there's somewhere *else* that `pod update` is looking? Perhaps it
> has a cache of some kind that it fails to update?)

macos/Podfile

@@ -1,6 +1,6 @@
 # This should match the macOS Deployment Target
 # (in Xcode, that's in project > Runner > Info)
-platform :osx, '10.14'
+platform :osx, '10.15'
 
 # CocoaPods analytics sends network stats synchronously affecting flutter build latency.
 ENV['COCOAPODS_DISABLE_STATS'] = 'true'

macos/Podfile.lock

@@ -183,6 +183,6 @@ SPEC CHECKSUMS:
   video_player_avfoundation: 7c6c11d8470e1675df7397027218274b6d2360b3
   wakelock_plus: 4783562c9a43d209c458cb9b30692134af456269
 
-PODFILE CHECKSUM: 373340d0a0047cc0734b3222969df87760de28c0
+PODFILE CHECKSUM: bcc9d1ecd86955a8f8907c4c114e92d74ab88cc1
 
 COCOAPODS: 1.16.2

macos/Runner.xcodeproj/project.pbxproj

@@ -422,7 +422,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				MACOSX_DEPLOYMENT_TARGET = 10.14;
+				MACOSX_DEPLOYMENT_TARGET = 10.15;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = macosx;
 				SWIFT_COMPILATION_MODE = wholemodule;
@@ -501,7 +501,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				MACOSX_DEPLOYMENT_TARGET = 10.14;
+				MACOSX_DEPLOYMENT_TARGET = 10.15;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				ONLY_ACTIVE_ARCH = YES;
 				SDKROOT = macosx;
@@ -548,7 +548,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				MACOSX_DEPLOYMENT_TARGET = 10.14;
+				MACOSX_DEPLOYMENT_TARGET = 10.15;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = macosx;
 				SWIFT_COMPILATION_MODE = wholemodule;