Align docs with current IAP SDK versions (iOS 1.6.5, Android 1.6.8)

The podspec and gradle.properties were bumped to iOS `1.6.5` and
Android `1.6.8`, but the README, get-started guide, and changelog
were never updated to match.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Armaxis

README.md

@@ -5,8 +5,8 @@
 The In-App Payments plugin for Square [In-App Payments SDK] is a wrapper for the native Android and iOS SDKs and 
 supports the following native In-App Payments SDK versions:
 
-  * iOS: `1.6.3`
-  * Android: `1.6.6`
+  * iOS: `1.6.5`
+  * Android: `1.6.8`
 
 ## Additional documentation
 

docs/get-started.md

@@ -139,7 +139,7 @@ are just examples.
           compileSdkVersion = 33
           targetSdkVersion = 33
           supportLibVersion = "33.0.0"
-          sqipVersion="1.6.6"
+          sqipVersion="1.6.8"
       }
       ...
     }