deps: Update flutter_lints to 6.0.0, from 5.0.0

Changelog:
  https://pub.dev/packages/flutter_lints/changelog#600

Also includes changes to fix lint failures for the new
'unnecessary_underscores' lint, probably to encourage using
wildcard variable `_`:
  https://dart.dev/language/variables#wildcard-variables

Without this change `flutter analyze` reports the following:

   info • Unnecessary use of multiple underscores • lib/widgets/autocomplete.dart:133:40 • unnecessary_underscores
   info • Unnecessary use of multiple underscores • lib/widgets/autocomplete.dart:156:38 • unnecessary_underscores
   info • Unnecessary use of multiple underscores • lib/widgets/autocomplete.dart:156:42 • unnecessary_underscores
   info • Unnecessary use of multiple underscores • lib/widgets/emoji_reaction.dart:333:34 • unnecessary_underscores

Author: rajveermalviya

lib/widgets/autocomplete.dart

@@ -130,7 +130,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
       //   `optionsBuilder` we pass is just a function that returns
       //   _resultsToDisplay, which is computed with lots of help from
       //   AutocompleteView.
-      optionsViewBuilder: (context, _, __) {
+      optionsViewBuilder: (context, _, _) {
         return Align(
           alignment: Alignment.bottomLeft,
           child: Material(
@@ -153,7 +153,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
       // The third isn't helpful; it lets us opt into behavior we don't actually
       // want (see discussion:
       //   <https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/autocomplete.20UI/near/1599994>)
-      fieldViewBuilder: (context, _, __, ___) => widget.fieldViewBuilder(context),
+      fieldViewBuilder: (context, _, _, _) => widget.fieldViewBuilder(context),
     );
   }
 }

lib/widgets/emoji_reaction.dart

@@ -330,7 +330,7 @@ class _ImageEmoji extends StatelessWidget {
       // Unicode and text emoji get scaled; it would look weird if image emoji didn't.
       textScaler: _squareEmojiScalerClamped(context),
       emojiDisplay: emojiDisplay,
-      errorBuilder: (context, _, __) => _TextEmoji(
+      errorBuilder: (context, _, _) => _TextEmoji(
         emojiDisplay: TextEmojiDisplay(emojiName: emojiName), selected: selected),
     );
   }

pubspec.lock

@@ -441,10 +441,10 @@ packages:
     dependency: "direct dev"
     description:
       name: flutter_lints
-      sha256: "5398f14efa795ffb7a33e9b6a08798b26a180edac4ad7db3f231e40f82ce11e1"
+      sha256: "3105dc8492f6183fb076ccf1f351ac3d60564bff92e20bfc4af9cc1651f4e7e1"
       url: "https://pub.dev"
     source: hosted
-    version: "5.0.0"
+    version: "6.0.0"
   flutter_localizations:
     dependency: "direct main"
     description: flutter
@@ -682,10 +682,10 @@ packages:
     dependency: transitive
     description:
       name: lints
-      sha256: c35bb79562d980e9a453fc715854e1ed39e24e7d0297a880ef54e17f9874a9d7
+      sha256: a5e2b223cb7c9c8efdc663ef484fdd95bb243bff242ef5b13e26883547fce9a0
       url: "https://pub.dev"
     source: hosted
-    version: "5.1.1"
+    version: "6.0.0"
   logging:
     dependency: transitive
     description:

pubspec.yaml

@@ -98,7 +98,7 @@ dev_dependencies:
   drift_dev: ^2.5.2
   fake_async: ^1.3.1
   flutter_checks: ^0.1.2
-  flutter_lints: ^5.0.0
+  flutter_lints: ^6.0.0
   ini: ^2.1.0
   json_serializable: ^6.5.4
   legacy_checks: ^0.1.0