[in_app_purchase] Updates min SDK to 3.29 (flutter#9825)

For the `in_app_purchase` packages:
- Updates the min SDK version to 3.29
- Runs the autoformatter with the new format
- Update code excerpts

This does not update versions because pushing format changes (even in READMEs) isn't worth doing a release that drops an SDK version (which we don't normally version)

Prep for flutter#9816

Author: stuartmorgan-g

packages/in_app_purchase/in_app_purchase/CHANGELOG.md

@@ -1,6 +1,6 @@
 ## NEXT
 
-* Updates minimum supported SDK version to Flutter 3.27/Dart 3.6.
+* Updates minimum supported SDK version to Flutter 3.29/Dart 3.7.
 
 ## 3.2.3
 * Updates minimum `in_app_purchase_storekit` version to 0.4.0.