Allocation of Resources Without Limits or Throttling

Summary

@fastify/accepts-serializer cached serializer-selection results keyed by the request Accept header without a size limit or eviction policy. A remote unauthenticated client could send many distinct but matching Accept header variants to make the cache grow unbounded, eventually exhausting the Node.js heap and crashing the process. Versions through 6.0.3 are affected by this issue. Update to 6.0.4 or later, which bounds the cache via an LRU with a default size of 100 entries, configurable through the new `cacheSize` plugin option.