PHP 8.2 updates

[cviebrock]

Some fixes for PHP 8.2, and code-quality improvements:

• add PHPstan and Rector to quality tooling
• switch to php-cs-fixer for linting
• convert "MyClass" to MyClass::class
• other fixes: various typos, code smells, and other issues identified by phpstan/rector

Note: the return value for the following methods has changed, so it will likely mean updates to projects and other packages that rely on Admin:

• AdminDBConfirmation::processDBData(): void
• AdminEdit::validate(): bool
• AdminConfirmation::prorcessResponse(): bool

Note: because of the above, the logic in 66e8fbf needed a refactor. Please confirm that my approach makes sense (i.e. a null would have been treated like false in a comparison check, so returning false makes sense in that context).