[PHP 8.2] Fix deprecated string interpolation patterns

PHP 8.2 deprecates string interpolation patterns that place the
dollar sign outside the curly braces. This fixes such patterns
by replacing them with proper curly braced patterns.

See:
 - [PHP.Watch: `${var}` string interpolation deprecated](https://php.watch/versions/8.2/$%7Bvar%7D-string-interpolation-deprecated)
 - [wiki.php.net RFC](https://wiki.php.net/rfc/deprecate_dollar_brace_string_interpolation)

src/SpdxLicenses.php

@@ -309,10 +309,10 @@ private function isValidLicenseString($license)
     (?<idstring>[\pL\pN.-]{1,})
 
     # license-id: taken from list
-    (?<licenseid>${licenses})
+    (?<licenseid>{$licenses})
 
     # license-exception-id: taken from list
-    (?<licenseexceptionid>${exceptions})
+    (?<licenseexceptionid>{$exceptions})
 
     # license-ref: [DocumentRef-1*(idstring):]LicenseRef-1*(idstring)
     (?<licenseref>(?:DocumentRef-(?&idstring):)?LicenseRef-(?&idstring))