Log if we ever gc.freeze() (#19440)

Spawning from
https://github.com/element-hq/synapse-small-hosts/issues/348 where some
test appears to be flaky because some homeserver objects are frozen in
the garbage collector.

We set
[`freeze=False`](https://github.com/element-hq/synapse-small-hosts/blob/a9a6869aa9a67176bdddc3b8ae2d0de0996d8cf4/multi_synapse/app/shard.py#L319-L321)
in the [Synapse Pro for small
hosts](https://docs.element.io/latest/element-server-suite-pro/synapse-pro-for-small-hosts/overview/)
code but I just want to use this log to make extra sure this isn't being
run somehow. The follow-up here would be to see what else would cause
something to be frozen in the garbage collector.

changelog.d/19440.misc

@@ -0,0 +1 @@
+Add log to explain when and why we freeze objects in the garbage collector.

synapse/app/_base.py

@@ -776,6 +776,11 @@ async def start(hs: "HomeServer", *, freeze: bool = True) -> None:
         #
         # PyPy does not (yet?) implement gc.freeze()
         if hasattr(gc, "freeze"):
+            logger.info(
+                "garbage collector: Freezing all allocated objects in the hopes that (almost) "
+                "everything currently allocated are things that will be used by the homeserver "
+                "for the rest of time. Doing so means less work each GC (hopefully)."
+            )
             gc.collect()
             gc.freeze()