Use minimalistic post-install upgrade message

[ivoanjo]

What does this PR do?

This PR is a follow-up on #3723.

For the 1.23.3 dd-trace-rb release, we included a quite detailed post-install message to tell customers that the new release was out:

Thank you for installing ddtrace. We have released our next major version!

As of version 2, `ddtrace` gem has been renamed to `datadog`.
The 1.x series will now only receive maintenance updates for security and critical bug fixes.

To upgrade, please replace gem `ddtrace` with gem `datadog`.
For detailed instructions on migration, see: https://dtdg.co/ruby-v2-upgrade

My thinking is -- some customers may not be able to move to 2.x as quickly as they wanted, and thus seeing that message again and again may start to be slightly annoying.

Thus, this PR replaces with with the minimalistic

The ddtrace gem has been renamed to datadog. We recommend upgrading: https://dtdg.co/ruby-v2-upgrade

Motivation:

Continue suggesting to customers that the should upgrade, while at the same time not being too annoying about it.

Additional Notes:

It actually may not a half-bad idea that we released a version with the big message, and then shortened it.

Specifically, customers that mostly keep up-to-date will probably see that message quite quickly, and thus upgrade.

For the long tail of customers that may need to remain on 1.x, the more minimalistic message that will go out in a few weeks/months on a future 1.x maintenance release will continue to remind them.

How to test the change?

$ bundle exec rake build
 # ...

ddtrace 1.23.3 built to pkg/ddtrace-1.23.3.gem.

$ gem install pkg/ddtrace-1.23.3.gem
Building native extensions. This could take a while...
    The ddtrace gem has been renamed to datadog. We recommend upgrading: https://dtdg.co/ruby-v2-upgrade

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.25%. Comparing base (4e33388) to head (545ff08).

Additional details and impacted files

@@             Coverage Diff             @@
##           1.x-stable    #3756   +/-   ##
===========================================
  Coverage       98.25%   98.25%           
===========================================
  Files            1257     1257           
  Lines           74916    74916           
  Branches         3566     3566           
===========================================
  Hits            73607    73607           
  Misses           1309     1309           

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[ivoanjo]

Thanks y'all :)

I've applied Marco's latest suggestion, and Tony also acked that one, so let's get this show on the road!