Require Ruby 2.5+ for libdatadog Ruby gem

This matches what dd-trace-rb is using
([git](https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/version.rb#L14),
[rubygems](https://rubygems.org/gems/datadog)) and cannot cause
backwards compatibility issues because we pin dd-trace-rb versions
to libdatadog major.minor versions, so it's not possible for an
older version of the gem on Ruby 2.4 to try to use the latest
libdatadog.

Author: ivoanjo

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" 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"