[PROF-10015] Fix Phusion Passenger detection when not in Gemfile/gems.rb

[ivoanjo]

What does this PR do?

In #2978 because of an incompatibility with Phusion Passenger, we added autodetection code to enable the "no signals" workaround when this web server is in use.

This code was later extended in #3280 once an upstream fix was released, so that the "no signals" workaround was only applied on old Passenger versions.

But in #3721 we got a report that our version detection code did not work correctly in setups where Passenger was not installed via Gemfile/gems.rb.

This PR fixes the version detection in these situations.

Motivation:

Make sure the "no signals" workaround does not get enabled in setups where it does not need to be.

Additional Notes:

N/A

How to test the change?

I've added test coverage for this issue.

Additionally, here's how to manually install Passenger and validate that the "no signals" workaround does not get enabled:

$ cat Gemfile
source 'https://rubygems.org'

gem 'datadog', git: 'https://github.com/DataDog/dd-trace-rb.git', branch: 'ivoanjo/prof-10015-fix-passenger-detection'
gem 'rack'
$ cat config.ru
require 'datadog/profiling/preload'

app = ->(env) {
  puts "Running on passenger #{PhusionPassenger::VERSION_STRING.inspect}"
  [200, {}, ["Hello, World!"]]
}
run app

$ docker run --network=host -ti -v `pwd`:/working ruby:3.3 /bin/bash
$ cd working/
$ apt update && apt-get install -y dirmngr gnupg apt-transport-https ca-certificates curl
$ curl https://oss-binaries.phusionpassenger.com/auto-software-signing-gpg-key.txt | gpg --dearmor | tee /etc/apt/trusted.gpg.d/phusion.gpg >/dev/null
$ sh -c 'echo deb https://oss-binaries.phusionpassenger.com/apt/passenger bookworm main > /etc/apt/sources.list.d/passenger.list'
$ apt update && apt-get install -y nginx libnginx-mod-http-passenger
$ bundle install
$ DD_PROFILING_ENABLED=true /usr/bin/passenger start

in master this outputs the warning message

WARN -- datadog: [datadog] Enabling the profiling "no signals"
workaround because an incompatible version of the passenger gem
is installed. Profiling data will have lower quality.To fix this,
upgrade the passenger gem to version 6.0.19 or above.

With this fix, this message is not shown, confirming the "no signals" workaround is not enabled.