Compatibility with PHP 8

Nobody has claimed this yet.

### Preconditions
In PHP 8, functions like **token_get_all** treat namespaced names as single token, e.g.
```
// Before: T_NS_SEPARATOR T_STRING
// After: T_NAME_FULLY_QUALIFIED
```
Details:
https://wiki.php.net/rfc/namespaced_names_as_token

There are couple places in the code that rely on the php7-style implementation:
```
\Magento2\Sniffs\Exceptions\ThrowCatchSniff::getFullClassName
\Magento2\Sniffs\NamingConvention\ReservedWordsSniff::validateNamespace
```

### Expected result
These methods should be checked and updated, the logic should work on PHP 8 as it was on PHP 7.

example from the main repo where new approach is used together with the old one:
https://github.com/magento/magento2/blob/platform-health/setup/src/Magento/Setup/Module/Di/Code/Reader/FileClassScanner.php#L155

Start by reading ThrowCatchSniff::getFullClassName and ReservedWordsSniff::validateNamespace, focusing on their token handling and the PHP 8 namespaced-name token behavior described in the issue. Compare the referenced Magento approach, then verify both sniffs preserve PHP 7 behavior while working correctly with PHP 8 tokens.

Written by the indexing model from the issue text.