Allowed major version source_gen 3, Upgraded dependencies

[sumitsharansatsangi]

• Solved issue Allow source_gen major version 3 in generator #742.
• Upgraded Dart dependencies.
• Upgraded native dependencies to version 4.3.1.
• Minimum Dart SDK required is now 3.8.0.

[Mairramer]

Could you avoid running the formatter in this PR? This should be done later in a separate PR, applied only to the essential files. Including it here makes the review harder.

[sumitsharansatsangi]

@Mairramer , Could you review it now?

[greenrobot-team]

Resolved by #752 and #751 instead. So sorry, closing.

As I said, please talk to us before proposing larger changes (also for license reasons)!

I had a quick look regardless and I likely would not have accepted this anyway as this mixes various unrelated changes in just two commits (and to be argued in a single pull request).

Also note that @Mairramer is not a maintainer of this project. You can see that I am by the "Member" tag in my comments or when hovering over my user name.