**What does this PR do?**

Back in #3053, a customer reported that profiling was not working
on ruby-head (for Ruby 3.3).

At the time, I tracked it down to two issues:
1. A bunch of headers had changed, and thus we needed a new
   release of `debase-ruby_core_source` to include them
2. We were getting errors when loading the profiler due to the
   `ruby_current_ec` VM symbol no longer being available.

At the time, I chose to disable profiling on Ruby 3.3 (#3054) until
we could tackle these two items.

Item 1. was fixed in #3163, that pulls in a new release of
`debase-ruby_core_source` that includes the 3.3-preview2 headers.

This PR fixes item 2. and re-enables profiling on Ruby 3.3
(and thus reverts #3054).

**Motivation:**

Make sure we're in good shape for the Ruby 3.3 final release in
December.

**Additional Notes:**

We didn't use the magic `ruby_current_ec` VM symbol directly.
Instead, we were using `GET_RACTOR()` and `GET_EC()` which call
a few methods that are defined as `inline` in `vm_core.h`.

Those methods all relied on `ruby_current_ec`, and thus hiding
that symbol made them break.

As an alternative, we can get to the same objects by starting
with the current thread, and navigating the object graph a bit.

The `rb_thread_current()` also relies on `ruby_current_ec` but
crucially **is not inline** and is actually a public API of the VM,
so that one kept working.

I hesitated on if we should use `rb_thread_current()` on all
Rubies with Ractors, or if we should keep the fast path for
3.0 to 3.2. I ended up deciding to keep the fast path since
it was quite easy to do, but in the future we could instead
choose to simplify this whole thing.

P.s.: This is one of those PRs where a lot of work went into
a handful of unimpressive lines of code 😅 .

**How to test the change?**

Validate that CI is passing on Ruby 3.3!

Fixes #3053