Update dependencies to patch known CVEs

composer audit reported 37 advisories across 15 packages, including
high-severity ones in symfony/security-http. Ran composer update
within the existing 7.4.* constraints - composer audit now reports
zero advisories. Also adds symfony/rate-limiter, needed for login
throttling and invite-code rate limiting in the next commit.

Flex removed a stale, non-functional sendgrid notifier config left
over from before the app switched to Mailgun as part of the sync.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

symfony.lock

@@ -262,15 +262,6 @@
             "config/routes/security.yaml"
         ]
     },
-    "symfony/sendgrid-mailer": {
-        "version": "7.3",
-        "recipe": {
-            "repo": "github.com/symfony/recipes",
-            "branch": "main",
-            "version": "4.4",
-            "ref": "224aedffb66812dc2b0965dabc14d5f800941da6"
-        }
-    },
     "symfony/stimulus-bundle": {
         "version": "2.30",
         "recipe": {