Update memcached to 1.6.21 (#2231)

Bumps 3 years of patch releases !

https://github.com/memcached/memcached/wiki/ReleaseNotes

Author: williamdes

docker-compose.yml

@@ -110,7 +110,7 @@ services:
       - "sentry-smtp-log:/var/log/exim4"
   memcached:
     <<: *restart_policy
-    image: "memcached:1.6.9-alpine"
+    image: "memcached:1.6.21-alpine"
     command: ["-I", "${SENTRY_MAX_EXTERNAL_SOURCEMAP_SIZE:-1M}"]
     healthcheck:
       <<: *healthcheck_defaults