Update pubspec.yaml.tmpl build version tracker on line 5 (flutter#173600)

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Added a default +1 build number to the version field in
pubspec.yaml.tmpl (line 5). This ensures consistent versioning behavior
across iOS and Android for newly created Flutter projects.

Why this change?
- On iOS, CFBundleShortVersionString (marketing version) must be numeric
(major.minor.patch) and does not include the build number unless
explicitly set.
- On Android, the version name and version code can both be set,
allowing more flexibility in representing the build number.
- Without a build number in pubspec.yaml, Android defaults to +1 while
iOS omits it, resulting in inconsistent version strings between
platforms.

Benefits
- Consistency: Both iOS and Android builds will start with the same
format, e.g., 1.0.0+1.
- Clarity: New projects have a visible build number by default, reducing
confusion for new developers.
- Best Practice: Aligns with Apple’s numeric marketing version rules
while preserving Android’s flexibility.

Developers can still override --build-name and --build-number as needed
for custom versioning.
*List which issues are fixed by this PR. You must list at least one
issue. An issue is not required if the PR fixes something trivial like a
typo.*

*If you had to change anything in the [flutter/tests] repo, include a
link to the migration guide as per the [breaking change policy].*
Fixes flutter#165325

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---------

Co-authored-by: Ben Konyi <bkonyi@google.com>

Author: Spivak-adam

packages/flutter_tools/templates/app/pubspec.yaml.tmpl

@@ -2,7 +2,7 @@ name: {{projectName}}
 description: {{description}}
 {{#withEmptyMain}}
 publish_to: 'none'
-version: 0.1.0
+version: 0.1.0+1
 
 environment:
   sdk: {{dartSdkVersionBounds}}

packages/flutter_tools/test/commands.shard/permeable/create_test.dart

@@ -4752,6 +4752,19 @@ To keep the default AGP version $templateAndroidGradlePluginVersion, download a
     }, getCurrentDirectory: () => out);
     expect(logger.statusText, isNot(contains(r'  $ cd')));
   }, overrides: {Logger: () => logger});
+
+  testUsingContext('generated pubspec uses default build number (+1) for empty app', () async {
+    await projectDir.create(recursive: true);
+
+    await _createProject(
+      projectDir,
+      <String>['--no-pub', '--template=app', '--empty'],
+      <String>['lib/main.dart', 'pubspec.yaml'],
+    );
+
+    final String pubspec = await projectDir.childFile('pubspec.yaml').readAsString();
+    expect(pubspec, contains(RegExp(r'^version:\s*0\.1\.0\+1\s*$', multiLine: true)));
+  });
 }
 
 Future<void> _createProject(