[camera] Document -1 return value for getExposureOffsetStepSize (flutter#6182)

Documents `getExposureOffsetStepSize` to return -1 if the device does not support exposure compensation. Helps account for `camera_android_camerax` since CameraX does not return a step size if exposure compensation is not supported.

A follow up to [this discussion](flutter/packages#6059 (comment)).

Author: camsim99

packages/camera/camera_platform_interface/CHANGELOG.md

@@ -1,6 +1,8 @@
-## NEXT
+## 2.7.4
 
 * Updates minimum supported SDK version to Flutter 3.13/Dart 3.1.
+* Documents `getExposureOffsetStepSize` to return -1 if the device does not support
+  exposure compensation.
 
 ## 2.7.3
 

packages/camera/camera_platform_interface/lib/src/platform_interface/camera_platform.dart

@@ -221,7 +221,8 @@ abstract class CameraPlatform extends PlatformInterface {
 
   /// Gets the supported step size for exposure offset for the selected camera in EV units.
   ///
-  /// Returns 0 when the camera supports using a free value without stepping.
+  /// Returns 0 when the camera supports using a free value without stepping and
+  /// returns -1 when exposure compensation is not supported.
   Future<double> getExposureOffsetStepSize(int cameraId) {
     throw UnimplementedError('getMinExposureOffset() is not implemented.');
   }

packages/camera/camera_platform_interface/pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/flutter/packages/tree/main/packages/camera/camera
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+camera%22
 # NOTE: We strongly prefer non-breaking changes, even at the expense of a
 # less-clean API. See https://flutter.dev/go/platform-interface-breaking-changes
-version: 2.7.3
+version: 2.7.4
 
 environment:
   sdk: ^3.1.0