Update to latest billing sdk 6.0, update gradle and flutter sdk, add some features

[offline-first]

• offerTags on SubscriptionOfferAndroid now supported
• add freeTrialDays on PricingPhaseAndroid
• add periodToDays method
• add priceAmountMicros to PricingPhaseAndroid
• fix: price, formattedPrice and currency is null on oneTime purchase

[offline-first]

How to fix this Flutter CI build issue (update flutter/dart version)?

[hyochan]

How to fix this Flutter CI build issue (update flutter/dart version)?

I'll try to handle this. Sorry for the force-push

[codecov[bot]]

Codecov Report

Merging #435 (1fe08bf) into main (dd841c4) will decrease coverage by 1.02%. The diff coverage is 21.05%.

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
- Coverage   47.69%   46.68%   -1.02%     
==========================================
  Files           4        3       -1     
  Lines         478      482       +4     
==========================================
- Hits          228      225       -3     
- Misses        250      257       +7     

Impacted Files	Coverage Δ
lib/utils.dart	45.83% <0.00%> (-18.88%)	:arrow_down:
lib/modules.dart	46.66% <25.80%> (-0.30%)	:arrow_down:

... and 2 files with indirect coverage changes

[offline-first]

@hyochan can you give me a hint how to auto format with Android Studio (Hedgehog). I use option + command + L

[github-actions[bot]]

This PR is stale because it has been open 90 days with no activity. Leave a comment or this will be closed in 7 days