deps: Upgrade Flutter to latest main, 3.14.0-5.0.pre.23

And update Flutter's supporting libraries to match.

Also remove a `!` operator that Dart 3.2 identifies as redundant.
(Happily, the change seems to be controlled by the version specified
in our pubspec.yaml, not by the version of the Dart binary -- so a
newer Dart binary will accept our code both before and after this
change.)

Author: gnprice

lib/api/core.dart

@@ -64,7 +64,7 @@ class ApiConnection {
 
   void addAuth(http.BaseRequest request) {
     if (_authValue != null) {
-      request.headers['Authorization'] = _authValue!;
+      request.headers['Authorization'] = _authValue;
     }
   }
 

pubspec.lock

@@ -181,10 +181,10 @@ packages:
     dependency: "direct main"
     description:
       name: collection
-      sha256: f092b211a4319e98e5ff58223576de6c2803db36221657b46c82574721240687
+      sha256: ee67cb0715911d28db6bf4af1026078bd6f0128b07a5f66fb2ed94ec6783c09a
       url: "https://pub.dev"
     source: hosted
-    version: "1.17.2"
+    version: "1.18.0"
   convert:
     dependency: transitive
     description:
@@ -1078,5 +1078,5 @@ packages:
     source: hosted
     version: "3.1.2"
 sdks:
-  dart: ">=3.1.0-285.0.dev <4.0.0"
-  flutter: ">=3.12.0-15.0.pre.69"
+  dart: ">=3.2.0-73.0.dev <4.0.0"
+  flutter: ">=3.14.0-5.0.pre.23"

pubspec.yaml

@@ -24,8 +24,8 @@ environment:
   # that by the time we want to release, these will have become stable.
   # TODO: Before general release, switch to stable Flutter and Dart versions,
   #   or pin exact versions: https://github.com/zulip/zulip-flutter/issues/15
-  sdk: '>=3.1.0-285.0.dev <4.0.0'
-  flutter: '>=3.12.0-15.0.pre.69'
+  sdk: '>=3.2.0-73.0.dev <4.0.0'
+  flutter: '>=3.14.0-5.0.pre.23'
 
 # Dependencies specify other packages that your package needs in order to work.
 # To automatically upgrade your package dependencies to the latest versions