Fix PHP deprecations

[markfullmer]

Deprecated: Array and string offset access syntax with curly braces is deprecated in /app/vendor/writecrow/highlighter/src/HighlightExcerpt.php on line 173 Deprecated: Array and string offset access syntax with curly braces is deprecated in /app/vendor/writecrow/highlighter/src/HighlightExcerpt.php on line 174

[markfullmer]

Deprecated function: htmlentities(): Passing null to parameter #1 ($string) of type string is deprecated in writecrow\Highlighter\Highlighter::finalize() (line 354 of /vendor/writecrow/highlighter/src/Highlighter.php)

https://stackoverflow.com/questions/71707325/migration-to-php-8-1-how-to-fix-deprecated-passing-null-to-parameter-error-r