refactor!: use latest stable flutter version, use camel case for enum fields, add linter, fix lints

[josxha]

• remove git ingored pubspec.lock file
• clean up pubspec.yaml files (remove unused dependencies and generated code from project creation)
• add flutter_lints (if wanted, a more strict lint rule set can be used in the future, but flutter_lints gives a good base for now)
• fix lints
• [BREAKING] rename enum fields to camel case, add information to CHANGELOG.md
• enhance the usage of const and final variables
• remove a specific flutter version from the CI workflows to prevent an out-of-date version

All CI workflows passed in my fork.

[kuhnroyal]

TBH, I think this is too big to review. Can you make a PR just adding the lints and ignore them in the analysis_options.yaml? Afterwards we can create individual PRs to fix specific warnings.