Commit Message: Removes no-longer-needed Stats::Allocator interface, and renames the sole implementation, Stats::AllocatorImpl to Stats::Allocator.

A long time ago, all Envoy stats were stored in a large shared memory block, significantly limiting how many you could get, and the lengths of their names. This facilitated keeping stat continuity across parent/child hot-restart. The fixed shared-mem block size was very limiting and a bit inefficient, so an alternative mechanism of keeping stats on the heap was added for when hot-restart was not needed. To switch between them, we used the Allocator interface, and kept two implementations of that.

Then we removed the shared-memory version in favor of using message passing to update counters from parent to child during hot restart.

Now it's time to remove that interface; it's making it a bit harder to reason about guaranteeing that stats users don't call the allocator directly; it's really an implementation detail of the stats system. So this is a pure refactor (no functional change) which can help that, and make it easier to reason about the safety of #43958 .

As removing headers and class names can be a breaking change, temporary forwarding headers are left behind, temporarily; out-of-repo referencess should be changed by June.

Additional Description:
Risk Level: low -- pure. refactor
Testing: CI
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features:

Request variables

Key	Value
ref	ff131c6
sha	d50f726
pr	43968
base-sha	7f56aad
actor	@jmarantz
message	Stats: Remove the pure interface Stats::Allocator and rename the sole implementation from Stats::AllocatorImpl to Stats::Allocator...
started	1773673591.946021
target-branch	main
trusted	false

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Envoy version (as used in this CI run)