Remove superfluous callout of new resolver

The new resolver has been out for nearly three years now.

I don't think we need to highlight it on the Readme anymore.

Folks who are truly affected are far more likely to google their errors and then get redirected to it.

By removing the noise from the Readme, it stops distracting from other stuff.

Author: jeffwidman

README.rst

@@ -19,8 +19,6 @@ We release updates regularly, with a new version every 3 months. Find more detai
 * `Release notes`_
 * `Release process`_
 
-In pip 20.3, we've `made a big improvement to the heart of pip`_; `learn more`_. We want your input, so `sign up for our user experience research studies`_ to help us do it right.
-
 **Note**: pip 21.0, in January 2021, removed Python 2 support, per pip's `Python 2 support policy`_. Please migrate to Python 3.
 
 If you find bugs, need help, or want to talk to the developers, please use our mailing lists or chat rooms:
@@ -49,9 +47,6 @@ rooms, and mailing lists is expected to follow the `PSF Code of Conduct`_.
 .. _Release process: https://pip.pypa.io/en/latest/development/release-process/
 .. _GitHub page: https://github.com/pypa/pip
 .. _Development documentation: https://pip.pypa.io/en/latest/development
-.. _made a big improvement to the heart of pip: https://pyfound.blogspot.com/2020/11/pip-20-3-new-resolver.html
-.. _learn more: https://pip.pypa.io/en/latest/user_guide/#changes-to-the-pip-dependency-resolver-in-20-3-2020
-.. _sign up for our user experience research studies: https://pyfound.blogspot.com/2020/03/new-pip-resolver-to-roll-out-this-year.html
 .. _Python 2 support policy: https://pip.pypa.io/en/latest/development/release-process/#python-2-support
 .. _Issue tracking: https://github.com/pypa/pip/issues
 .. _Discourse channel: https://discuss.python.org/c/packaging