Bump gradle from 7.0.2 to 7.3.0 in /flutter/android

Bumps gradle from 7.0.2 to 7.3.0.

---
updated-dependencies:
- dependency-name: com.android.tools.build:gradle
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

flutter/android/build.gradle

@@ -7,7 +7,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:7.0.2'
+        classpath 'com.android.tools.build:gradle:7.3.0'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
         classpath 'com.google.gms:google-services:4.3.3'
     }