[PROF-11524] Bump minimum Ruby version for libdatadog ruby gem + fix Ruby testing github action (#1080)

# What does this PR do?

This PR fixes the [Ruby gem scaffolding testing](https://github.com/DataDog/libdatadog/actions/workflows/test-ruby.yml) GitHub Action that was broken by #1067.

Specifically, #1067 introduced a new required dependency for running our scaffolding (but not the Ruby gem itself -- it's only for development) that only worked on modern Rubies.

Thus, older Rubies [started failing in CI with](https://github.com/DataDog/libdatadog/actions/runs/15305108278/job/43055672220): 

```
Could not find compatible versions

Because rubygems-await >= 0.3.0 depends on RubyGems >= 3.2
  and rubygems-await < 0.3.0 depends on RubyGems >= 3.4,
  RubyGems >= 3.2 is required.
So, because current RubyGems version is = 3.1.6,
  version solving has failed.
```

This PR fixes that issue by making that dependency optional on older Rubies...

...and while I was in the neighbourhood, I then:

1. Dropped support for Ruby < 2.5 from our library. See individual commit messages for more details on why this is OK
2. Dropped testing for Ruby < 2.5 from CI, and added a few more Ruby versions that came out since I first added this testing

# Motivation

Fix broken CI.

# Additional Notes

[The GHA action](https://github.com/DataDog/libdatadog/actions/workflows/test-ruby.yml) was previously also broken because of "Startup failure"s, that I fixed by going into the repository settings and making sure the required dependencies for this action are in the allowed list.

# How to test the change?

[This run of the action shows the new Ruby versions + everything green](https://github.com/DataDog/libdatadog/actions/runs/15305667598).

Author: ivoanjo

.github/workflows/test-ruby.yml

@@ -10,32 +10,11 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        ruby: [2.1, 2.3, 2.4, 2.5, 2.6, 2.7, '3.0', 3.1, head]
+        ruby: ['2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', head]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # 4.2.2
-    - uses: ruby/setup-ruby@922ebc4c5262cd14e07bb0e1db020984b6c064fe # 1.226.0
-      with:
-        ruby-version: ${{ matrix.ruby }}
-        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
-        working-directory: ruby/
-    - run: cd ruby && bundle exec rake
-  # Workaround for broken issue
-  # ```
-  # ERROR:  While executing gem ... (RuntimeError)
-  # Marshal.load reentered at marshal_load
-  # ```
-  # suggested in https://github.com/ruby/setup-ruby/issues/496 (use ubuntu 20.04 instead of a later version)
-  test-legacy-workaround:
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ubuntu-20.04]
-        ruby: [2.2]
-    runs-on: ${{ matrix.os }}
-    steps:
-    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # 4.2.2
-    - uses: ruby/setup-ruby@922ebc4c5262cd14e07bb0e1db020984b6c064fe # 1.226.0
+    - uses: ruby/setup-ruby@13e7a03dc3ac6c3798f4570bfead2aed4d96abfb # 1.244.0
       with:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically

ruby/gems.rb

@@ -7,7 +7,7 @@
 gem "rake", ">= 12.0", "< 14"
 gem "rspec", "~> 3.10"
 gem "standard", "~> 1.7", ">= 1.7.2" unless RUBY_VERSION < "2.6"
-gem "http", "~> 5.0" unless RUBY_VERSION < "2.5"
+gem "http", "~> 5.0"
 gem "pry"
 gem "pry-byebug" unless RUBY_VERSION > "3.1"
-gem "rubygems-await"
+gem "rubygems-await" unless RUBY_VERSION < "3.1"

ruby/libdatadog.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
     "libdatadog is a Rust-based utility library for Datadog's ddtrace gem."
   spec.homepage = "https://docs.datadoghq.com/tracing/"
   spec.license = "Apache-2.0"
-  spec.required_ruby_version = ">= 2.1.0"
+  spec.required_ruby_version = ">= 2.5.0"
 
   spec.metadata["allowed_push_host"] = "https://rubygems.org"