[PHP 8.2] Fix `${var}` string interpolation deprecation

[Ayesh]

Summary

PHP 8.2 deprecates "${var}" string interpolation pattern. This fixes all three of such occurrences in braintree/braintree_php package.

• PHP 8.2: ${var} string interpolation deprecated
• RFC

Checklist

• [x] Added changelog entry
• [x] Ran unit tests (Check the README for instructions)
• [x] I understand that unless this is a Draft PR or has a DO NOT MERGE label, this PR is considered to be in a deploy ready state and can be deployed if merged to main

[hollabaq86]

👋 @Ayesh thanks for the PR! We'll take a look and provide feedback. For internal tracking, issue 358

[saralvasquez]

This has been added and will be in the next release of the SDK. Thanks for the PR!