Increase minimum polyfill version

This is an attempt to fix a test failure which seems unrelated to this change,
which seems to require str_ends_with() somewhere.

Author: fredden

composer.json

@@ -15,7 +15,7 @@
         "phpcompatibility/php-compatibility": "^9.3",
         "squizlabs/php_codesniffer": "^3.6.1",
         "rector/rector": "^0.14.8",
-        "symfony/polyfill": "^1.15"
+        "symfony/polyfill": "^1.16"
     },
     "require-dev": {
         "phpunit/phpunit": "^9.5.8"