Make Dart pre-commit hook compatible with the latest Dart SDKs

Dart introduced sound null safety in version 2.12.0, and as of Dart 3,
null safety is mandatory. Older Dart SDKs allowed both pre-null safety
and null-safe packages, but modern Dart SDKs, where most source code is
null-safe, now reject pre-null safety packages.

The current `pubspec.yaml` template with `sdk: '>=2.10.0'` is
incompatible with recent Dart SDKs, leading to the following error:

An unexpected error has occurred: CalledProcessError: command: ('/path/to/dart-sdk/bin/dart', 'pub', 'get')
return code: 65
stdout:
    Resolving dependencies...
stderr:
    The lower bound of "sdk: '>=2.10.0'" must be 2.12.0'
    or higher to enable null safety.

    The current Dart SDK (3.8.3) only supports null safety.

    For details, see https://dart.dev/null-safety

To ensure compatibility with the modern Dart ecosystem, this commit
updates the minimum Dart SDK version to 2.12.0 or higher,
which implicitly supports null safety.
Additionally, `testing/get-dart.sh` has been updated to verify that
the pre-commit hook functions correctly with the latest Dart versions.

tests/languages/dart_test.py

@@ -10,7 +10,7 @@ from testing.language_helpers import run_language
 def test_dart(tmp_path):
     pubspec_yaml = '''\
 environment:
-  sdk: '>=2.10.0 <3.0.0'
+  sdk: '>=2.12.0 <4.0.0'
 
 name: hello_world_dart