fix: update Java version to 21 in build workflow and ensure workflow file is tracked on push and pull request events

Author: gopi2401

.github/workflows/build.yml

@@ -3,13 +3,15 @@ name: Build
 on:
   push:
     paths:
+      - ".github/workflows/build.yml"
       - "android/**"
       - "lib/**"
       - "pubspec.yaml"
     branches:
       - main
   pull_request:
     paths:
+      - ".github/workflows/build.yml"
       - "android/**"
       - "lib/**"
       - "pubspec.yaml"
@@ -30,7 +32,7 @@ jobs:
       # Setup Java environment for Android build
       - uses: actions/setup-java@v4
         with:
-          java-version: "17"
+          java-version: "21"
           distribution: "adopt"
           cache: "gradle"