bump to linter 1.21.1

Fixes: https://github.com/flutter/flutter/issues/100297

Change-Id: I240d8bfd77f48830b91ed80329dc6a2352e1cb8c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237724
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

.dart_tool/package_config.json

@@ -415,7 +415,7 @@
       "name": "linter",
       "rootUri": "../third_party/pkg/linter",
       "packageUri": "lib/",
-      "languageVersion": "2.12"
+      "languageVersion": "2.15"
     },
     {
       "name": "lints",

CHANGELOG.md

@@ -108,8 +108,11 @@
 
 #### Linter
 
-Updated the Linter to `1.20.0`, which includes changes that
+Updated the Linter to `1.21.1`, which includes changes that
 
+- updates `unnecessary_null_checks` and 
+  `null_check_on_nullable_type_parameter` to handle
+  list/set/map literals, and `yield` and `await` expressions.
 - fixes `unnecessary_null_aware_assignments` property-access
   false positives.
 - adds new lint: `use_super_parameters`.

DEPS

@@ -124,7 +124,7 @@ vars = {
   "intl_tag": "9669926609e7efc17dfd74fbb44ec719a7e573cc", # 0.17.0-nullsafety
   "jinja2_rev": "2222b31554f03e62600cd7e383376a7c187967a1",
   "json_rpc_2_rev": "7e00f893440a72de0637970325e4ea44bd1e8c8e",
-  "linter_tag": "ed26087ae1b54fb8c532fcda618e6bf08827e315", # 1.20.0
+  "linter_tag": "bced222d3132a1fd883f5ef2b9bb833a1131da8d", # 1.21.1
   "lints_tag": "8294e5648ab49474541527e2911e72e4c5aefe55", #2.0.0
   "logging_rev": "dfbe88b890c3b4f7bc06da5a7b3b43e9e263b688",
   "markupsafe_rev": "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",