[Linux] Update doctor wording for Linux (#99700)

Removes the beta-quality wording for Linux from messages in the tool.

Issue: https://github.com/flutter/flutter/issues/99697

packages/flutter_tools/lib/src/features.dart

@@ -108,7 +108,7 @@ const Feature flutterMacOSDesktopFeature = Feature(
 
 /// The [Feature] for Linux desktop.
 const Feature flutterLinuxDesktopFeature = Feature(
-  name: 'beta-quality support for desktop on Linux',
+  name: 'support for desktop on Linux',
   configSetting: 'enable-linux-desktop',
   environmentOverride: 'FLUTTER_LINUX',
   extraHelpText: 'Newer beta versions are available on the beta channel.',

packages/flutter_tools/test/general.shard/features_test.dart

@@ -96,7 +96,7 @@ void main() {
 
     testWithoutContext('Flutter Linux desktop help string', () {
       expect(flutterLinuxDesktopFeature.generateHelpMessage(),
-      'Enable or disable beta-quality support for desktop on Linux. '
+      'Enable or disable support for desktop on Linux. '
       'This setting will take effect on the master, beta, and stable channels. '
       'Newer beta versions are available on the beta channel.');
     });