**What does this PR do?**

This PR fixes an issue that we spotted in #8: that the logic we
inherited for matching on header folders for beta versions of Ruby
was a bit buggy.

Specifically, for preview/rc versions of Ruby, the "preview"/"rc"
part is (as far as I know) only part of the `RUBY_DESCRIPTION` and
does not show up anywhere else (including the `RUBY_VERSION`).

This meant that as we are shipping for instance headers for
* `ruby-3.4.0-preview2`
* `ruby-3.4.0-rc1`

The `RUBY_VERSION` for Ruby 3.4.0-preview2 is 3.4.0, and thus the
fuzzy matching logic would pick the "latest" option out of the
multiple options for 3.4.0, which would mean the 3.4.0-rc1 headers
were used for 3.4.0-preview2.

The way `ruby_core_source` version matching works is in two passes:
1. It first checks if there's an exact hit
2. If not, it falls back to a fuzzy version matching logic

By including the `preview`/`rc` in the string, my change affects 1:
if there's an exact hit on headers (e.g. `preview2` on `preview2`),
then those get used.

**Motivation:**

This change is useful since the 3.4.0-preview2 headers are not
compatible with 3.4.0-rc1 and vice-versa; so this way we can
continue to support both versions until stable 3.4.0 is out.

**Additional Notes:**

My change does not touch the fuzzy matching, e.g. for `preview1`,
since we have no exact match, the latest ones (`-rc1`) will get used.

I believe that there's no use in being "too clever" here.
If we have ruby 3.3.5 headers and 3.4.0-preview2, and we're
in `preview1`, what's the right answer of what to pick?

I argue there's no right answer here, so I didn't touch this
part of the logic.

Two other notes:

1. `RUBY_PATCHLEVEL < 0` is only for beta releases, so this
   change is not expected to impact any other case

2. The `$stderr.puts` is hidden by default during installation, so
   it's fine to leave like that (there's other similar prints in
   the profiler's `extconf.rb`)

**How to test the change?**

I've tested this manually with dd-trace-rb in different Ruby
versions.

```
$ ruby -v
ruby 3.4.0rc1 (2024-12-12 master 29caae9991) +PRISM [x86_64-linux]
Using datadog-ruby_core_source headers from datadog-ruby_core_source/lib/datadog/ruby_core_source/ruby-3.4.0-rc1

$ ruby -v
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +PRISM [x86_64-linux]
Using datadog-ruby_core_source headers from datadog-ruby_core_source/lib/datadog/ruby_core_source/ruby-3.4.0-preview2

$ ruby -v
ruby 3.4.0preview1 (2024-05-16 master 9d69619623) [x86_64-linux]
Using datadog-ruby_core_source headers from datadog-ruby_core_source/lib/datadog/ruby_core_source/ruby-3.4.0-rc1
```