Array and string offset access syntax with curly braces is deprecated

Problem/Motivation

The current code in the Symfony YAML Unescaper class is not compatible with PHP 7.4 due to the use of curly braces for string offset access. This has been deprecated in PHP 7.4 and will result in errors when running the code on newer PHP versions.

Steps to reproduce

1. Install PHP 7.4 or higher.
2. Use the Symfony YAML Unescaper class to unescape characters in a YAML string.
3. Observe the deprecated error caused by the use of curly braces for string offset access.

Proposed resolution

Update the code in the Symfony YAML Unescaper class to replace the deprecated curly braces with square brackets for string offset access, as shown in the provided patch.

Remaining tasks

• Review and apply the provided patch.
• Run tests to ensure compatibility with PHP 7.4 and higher.

User interface changes

None

API changes

None

Data model changes

None

Thanks!