Remove curly brace syntax for accessing array elements and string offsets

Reference: https://wiki.php.net/rfc/deprecate_curly_braces_array_access

All accesses to array elements and string offsets now show a deprecation message:

PHP Deprecated: Array and string offset access syntax with curly braces is deprecated in ...

This issue is about to remove all them, because they will stop working on php 8.0

Note that there is a script aiming to find uses (and fix them). It can be used for verifications, but surely worth checking all changes, spreading them to 3rd party libs when possible:

https://gist.github.com/theodorejb/763b83a43522b0fc1755a537663b1863