Upgrade AGP to 8.9.1 (#10458)

Upgrades AGP to 8.9.1.  A few dependabot PRs are stuck because they are failing with similar error messages:

```
* What went wrong:
Execution failed for task ':app:checkDebugAarMetadata'.
> A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
   > An issue was found when checking AAR metadata:

       1.  Dependency 'androidx.browser:browser:1.9.0' requires Android Gradle plugin 8.9.1 or higher.

           This build currently uses Android Gradle plugin 8.6.0.
```

Blocked PRs: 
* #9855
* #10045

## Pre-Review Checklist

Author: mboetger

.ci/legacy_project/README.md

@@ -41,12 +41,12 @@ and then deleting everything but `android/` from it:
   `build.gradle` to maintain compatibility with plugins that use
   Flutter's most recently supported API version.
 - Modifies `gradle-wrapper.properties` to upgrade the Gradle version
-  from 8.4 to 8.7. If a user runs into an error with the Gradle
+  from 8.4 to 8.11.1. If a user runs into an error with the Gradle
   version, the warning is clear on how to upgrade the version to
   one that we support.
 - Modifies `settings.gradle` to upgrade the Android Gradle Plugin (AGP)
   from version 8.3.0 (originally set in `build.gradle`; see bullet below)
-  to 8.6.0. If a user runs into an error with the AGP version, the warning
+  to 8.9.1. If a user runs into an error with the AGP version, the warning
   is clear on how to upgrade the version to one that we support.
 - Refactor plugin to use declarative Gradle apply instead of the
   imperative apply (this includes moving where the Android Gradle

.ci/legacy_project/all_packages/android/gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.7-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.11.1-all.zip

.ci/legacy_project/all_packages/android/settings.gradle

@@ -19,7 +19,7 @@ pluginManagement {
 // See https://github.com/flutter/flutter/blob/master/docs/ecosystem/Plugins-and-Packages-repository-structure.md#gradle-structure for more info.
 plugins {
     id "dev.flutter.flutter-plugin-loader" version "1.0.0"
-    id "com.android.application" version "8.6.0" apply false
+    id "com.android.application" version "8.9.1" apply false
     id "org.jetbrains.kotlin.android" version "2.1.0" apply false
 }