Feat/upgrade-analyzer8

[dorklein]

NOTE: this PR builds on top of the changes from "Support analyzer: ^7 and source_gen: ^2 and Freezed ^3" #41

• Updated version to 1.3.0 in pubspec.yaml files
• Upgraded packages to support analyzer ^8 and set minimum SDK to 3.8.0
• Updated CHANGELOG to reflect changes

Fixes #<issue_number_goes_here>

It's a good idea to open an issue first for discussion.

• [x] Tests pass
• [x] Appropriate changes to README are included in PR