**What does this PR do?**

For a while now, whenever the GC and more importantly the allocation
profiling features were enabled, we logged a user-visible message that
if Ractors were used, a bug in Ruby could cause the profiler to miss
data (due to https://bugs.ruby-lang.org/issues/19112 ).

The upstream issue has been fixed in
<ruby/ruby#15468> (some discussion as well
in <ruby/ruby#7184> ).

Thus, this PR does a few small things:

* Update the logged messages to mention the bug doesn't happen on
  Ruby 4

* Moves all messages to be debug-level messages (our logic already
  made sure they were only printed on Ruby 3, hah turns out we
  predicted this!)

  In particular our concern in the past for making the message
  user-visible by default was that customers might use Ractors and
  run into this issue. As it turns out that in Ruby 3 ractors were
  always experimental and had a number of bugs/issues, we don't
  expect anyone to run Ractors in production with Ruby 3. (Maybe
  this will change with Ruby 4?)

* Removes the `OnlyOnce` that was used to limit verboseness -- this
  is extra complexity that we don't need to maintain now that the
  log is debug level

**Motivation:**

Avoid annoying customers with log messages about Ractor support on
Ruby 3.

**Additional Notes:**

N/A

**How to test the change?**

For the warnings themselves, I've updated our tests to match the
new setup.

For the upstream issue, I manually used the testcase from
https://bugs.ruby-lang.org/issues/19112 and added prints to the
profiling GC and NEWOBJ tracepoints, confirming that on Ruby 3.4
they get disabled but on Ruby 4.0 they work fine.

I considered adding an explicit test for this but it seems a bit
brittle as it would be a test for something not happening after
Ractor GC which is kinda fishy to test for, so I ended up not doing
it.