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>

config/packages/notifier.yaml

@@ -1,13 +1,12 @@
-framework:
-    notifier:
-        chatter_transports:
-        texter_transports:
-            sendgrid: '%env(MAILER_DSN)%'
-        channel_policy:
-            # use chat/slack, chat/telegram, sms/twilio or sms/nexmo
-            urgent: ['email']
-            high: ['email']
-            medium: ['email']
-            low: ['email']
-        admin_recipients:
-            - { email: admin@example.com }
+framework:
+    notifier:
+        chatter_transports:
+        texter_transports:␍
+        channel_policy:
+            # use chat/slack, chat/telegram, sms/twilio or sms/nexmo
+            urgent: ['email']
+            high: ['email']
+            medium: ['email']
+            low: ['email']
+        admin_recipients:
+            - { email: admin@example.com }