Allow profiler to run without "no signals" workaround on passenger 6.0.19+

[ivoanjo]

What does this PR do?

This PR is the last step to fully fixing the issue reported in #2976.

In that issue, we discovered that he passenger web server had an incompatibility with the profiler. At the time, we added code to detect when passenger was in use and apply the "no signals" workaround to avoid this issue out-of-the-box.

Upstream passenger has since accepted our PR to fix the issue, allowing us in this PR to change our detection logic to:

a) Not enable the "no signals" workaround on passenger 6.0.19+
b) Provide an actionable error message to customers on older passenger versions to tell them that the best option is to upgrade

Motivation:

Improve the out-of-the-box experience of profiler users that use the passenger web server.

Additional Notes:

N/A

How to test the change?

I used the following config.ru:

def self.fib(n)
  n <= 1 ? n : fib(n-1) + fib(n-2)
end

app = ->(env) {
  puts " ** Got request!"

  [200, {}, ["Hello, World! #{fib(30)}"]]
}
run app

and Gemfile:

source 'https://rubygems.org'

gem 'ddtrace', path: 'path/to/local/repo'
 #gem 'passenger', '= 6.0.18'
gem 'passenger', '= 6.0.19'
gem 'puma'

to validate that the passenger versions are correctly detected + request processing is not impacted on either.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Fixes #2976

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b4c9019) 98.23% compared to head (80d5b61) 98.22%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3280      +/-   ##
==========================================
- Coverage   98.23%   98.22%   -0.01%     
==========================================
  Files        1253     1253              
  Lines       72393    72409      +16     
  Branches     3393     3395       +2     
==========================================
+ Hits        71112    71127      +15     
- Misses       1281     1282       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[AlexJF]

LGTM 👍