Updates to generating localized messages

[AdamVe]

The use of synthetic package is deprecated and will be removed, see this breaking change description. Instead apps should generate the localized messages into source and PR fixes this.

These changes will not affect the way how we work with localizations. When we need to add a new string, let's just put it into app_en.arb and build the app to be able to use it in code.

Changes:

1. Updated l10n.yaml with options to not use the synthetic package an pointed out where the new sources should be generated.
2. Updated all sources to include correct app_localizations.dart file.
3. the generated folder is ignored in git and will be always created during build, but for making sure that nothing is broken, the pre-commit hook is updated so that it always regenerates the localized messages sources.
4. AppLocalization.of(context) now returns no-nullable getter.
5. Updated license header years and added missing license headers.

This has been tested on desktop and Android on both Flutter 3.27.4 and 3.29.0 versions.