Remove unsupported ping_threshold option from mailer config to fix Symfony 8 cache:clear error

The ping_threshold option is not a valid framework.mailer configuration key in Symfony Mailer.
This caused composer install to fail during cache:clear on deployment.
The option should be set as a DSN query parameter on the MAILER_DSN environment variable instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/packages/mailer.yaml

@@ -1,4 +1,3 @@
 framework:
     mailer:
         dsn: '%env(MAILER_DSN)%'
-        ping_threshold: 10