add release notes

.release-notes/4531.md

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+## Make actor heap allocations more efficient by recycling freed memory
+
+Prior to this change, the actor heap garbage collection process would return freed memory back to the pony runtime at the end of a garbage collection run. The returning of memory to the pony runtime and allocating of new memory from the pony runtime are both expensive operations. This change makes it so that the actor garbage collection process keeps old freed memory chunks around with the expectation that the actor will very likely need memory again as it continues to run behaviors. This avoids the expensive return to and reallocation of memory from the pony runtime. It is possible that the overall application might end up using more memory as any freed memory chunks can only be reused by the actor that owns them and the runtime and other actors can no longer reuse that memory as they previously might have been able to.