Update Flutter wiki links (#6789)

Updates all links to the Flutter wiki to point to their new location in the flutter/flutter repository. (The sole exception is a link to a doc that doesn't have a final home yet, and is linked from legacy code anyway so doesn't really need to be updated.)

While touching the PR template, makes a few minor improvements:
- Removes the breaking change discussion that doesn't apply to this repository, as breaking changes are handled totally differently for packages (and is covered by the link to docs about Dart versioning).
- Adds text and a link to reflect the fact that some PRs can be changelog-exempt.

script/tool/lib/src/gradle_check_command.dart

@@ -133,7 +133,7 @@ class GradleCheckCommand extends PackageLoopingCommand {
   /// Documentation url for Artifact hub implementation in flutter repo's.
   @visibleForTesting
   static const String artifactHubDocumentationString =
-      r'https://github.com/flutter/flutter/wiki/Plugins-and-Packages-repository-structure#gradle-structure';
+      r'https://github.com/flutter/flutter/blob/master/docs/ecosystem/Plugins-and-Packages-repository-structure.md#gradle-structure';
 
   /// String printed as example of valid example root build.gradle repository
   /// configuration that enables artifact hub env variable.
@@ -158,7 +158,7 @@ class GradleCheckCommand extends PackageLoopingCommand {
     final RegExp keyPresentRegex =
         RegExp('$keyVariable' r"\s+=\s+'ARTIFACT_HUB_REPOSITORY'");
     final RegExp documentationPresentRegex = RegExp(
-        r'github\.com.*wiki.*Plugins-and-Packages-repository-structure.*gradle-structure');
+        r'github\.com.*flutter.*blob.*Plugins-and-Packages-repository-structure.*gradle-structure');
     final RegExp keyReadRegex =
         RegExp(r'if.*System\.getenv.*\.containsKey.*' '$keyVariable');
     final RegExp keyUsedRegex =
@@ -223,7 +223,7 @@ apply plugin: "com.google.cloud.artifactregistry.gradle-plugin"
   bool _validateArtifactHubSettingsUsage(
       RepositoryPackage example, List<String> gradleLines) {
     final RegExp documentationPresentRegex = RegExp(
-        r'github\.com.*wiki.*Plugins-and-Packages-repository-structure.*gradle-structure');
+        r'github\.com.*flutter.*blob.*Plugins-and-Packages-repository-structure.*gradle-structure');
     final RegExp artifactRegistryDefinitionRegex = RegExp(
         r'classpath.*gradle\.plugin\.com\.google\.cloud\.artifactregistry:artifactregistry-gradle-plugin');
     final RegExp artifactRegistryPluginApplyRegex = RegExp(