fix: fixed package versions

[monsieurtanuki]

What

• After the downgrade to flutter 3.0.5 (from 3.3.0), we're trying to use fixed versions of packages (in short, removing the starting ^ in the pubspec.yaml version entries)
• That is supposed to limit the impact of external package evolutions - currently we need stability in order to code our own bugs without the risk of being attacked by the others' ;)
• That's more a test than a lasting recommendation.
• Fun fact: flutter pub outdated still recommends new packages, even if we specify fixed version numbers (and implicitly state that we're not interested in new stuff)

Part of

• ⛔️ DO NOT release any version based on Flutter 3.3 #2919