PHP 8 - fatal error due to outdated Civix code using syntax no longer supported

Extension will not run under PHP8+ because old Civix code uses the {curly braces} string indexing notation, which has been removed. This results in a fatal error

Specifically on line 247 of membershipreport.civix.php.

Will raise PR to fix the specific issue, but the better fix is simply to regenerate this file with the latest version of Civix, I think.