Bump legacy all_packages project AGP version to 7.0.0, Gradle version…

… to 7.0.2 (#6591)

Upgrades legacy all_packages project AGP version to 7.0.0 and Gradle version to 7.0.2 as Flutter will begin enforcing supported versions for these Android dependencies.

This was prompted by dependabot AGP version upgrades like #6522 that are failing due to a what a clear warning* describes as a potential issue:

```
Warning: Flutter support for your project's Gradle version (6.7.1) will soon be dropped. Please upgrade your Gradle version to a version of at least 7.0.2 soon.
Alternatively, use the flag "--android-skip-build-dependency-validation" to bypass this check.

Potential fix: Your project's gradle version is typically defined in the gradle wrapper file. By default, this can be found at /b/s/w/ir/x/w/packages/legacy/all_packages/android/gradle/wrapper/gradle-wrapper.properties. 
For more information, see https://docs.gradle.org/current/userguide/gradle_wrapper.html.

Warning: Flutter support for your project's Android Gradle Plugin version (4.1.0) will soon be dropped. Please upgrade your Android Gradle Plugin version to a version of at least 7.0.0 soon.
Alternatively, use the flag "--android-skip-build-dependency-validation" to bypass this check.

Potential fix: Your project's AGP version is typically defined the plugins block of the `settings.gradle` file (/b/s/w/ir/x/w/packages/legacy/all_packages/android/settings.gradle), by a plugin with the id of com.android.application. 
If you don't see a plugins block, your project was likely created with an older template version. In this case it is most likely defined in the top-level build.gradle file (/b/s/w/ir/x/w/packages/legacy/all_packages/android/build.gradle) by the following line in the dependencies block of the buildscript: "classpath 'com.android.tools.build:gradle:<version>'".
```

Other dependabot upgrades that I believe are blocked by this change:
#6585
#6534
#6530
#6528
#6526

*Added in flutter/flutter#143341

.ci/legacy_project/README.md

@@ -42,3 +42,11 @@ and then deleting everything but `android/` from it:
 - Updates `gradle-wrapper.properties` from `6.7` to `6.7.1`, to add
   support for the Kotlin gradle plugin. If a user runs into this
   error, the error message is clear on how to upgrade.
+- Modifies `build.gradle` to upgrade the Android Gradle Plugin (AGP)
+  from version 4.1.0 to 7.0.0. 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.
+- Modifies `gradle-wrapper.properties` to upgrade the Gradle version
+  from 6.7.1 to 7.0.2. 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.

.ci/legacy_project/all_packages/android/build.gradle

@@ -5,7 +5,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:4.1.0'
+        classpath 'com.android.tools.build:gradle:7.0.0'
     }
 }
 

.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-6.7.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.0.2-all.zip