Migrate to null-safety

[devslava]

Currently this library can't be used with Flutter 2.5.0:

Error: Cannot run with sound null safety, because the following dependencies
    don't support null safety:
     - package:appbar_elevation

This PR migrates code to null-safety. To support it, Dart min version requirement should be 2.12.0.

[maherjaafar]

@amitkot any plans to merge this PR?

[maherjaafar]

You can use SliverAppBar with scrolledUnderElevation instead of this package.

Example code:

  CustomScrollView(
      slivers: [
        const SliverAppBar(
          title: Text('Dynamic elevation'),
          pinned: true,
          scrolledUnderElevation: 20,
        ),
        SliverList(
          delegate: SliverChildBuilderDelegate(
            (_, int index) {
              return ListTile(
                title: Text(index.toString()),
              );
            },
            childCount: 20,
          ),
        ),
      ],
    );

[amitkot]

@maherjaafar I no longer use this in my projects, the direction you suggest looks good!