Fixing old OpenMDAO build

[kanekosh]

Purpose

NOTE: RTD build is currently failing, and I believe this will fix the issue. This fix is not released yet so we'll wait for a while.

Fixed the build issue of the oldest OpenMDAO.

In the oldest build, we've been failing to build wheel for OpenMDAO (I don't know exactly why, but we don't need to care about it because the latest build doesn't have this error). Then, the previous versions of pip switch to setup.py install which works, but the latest pip deprecated it, hence we're seeing the error.
In this PR, I pinned the pip version for the oldest build so that it switches to the setup.py install method to build OM. This workaround does not affect the latest build.

Copied from an old build log:

 DEPRECATION: openmdao was installed using the legacy 'setup.py install' method, because a wheel could not be built for it. pip 23.1 will enforce this behaviour change. A possible replacement is to fix the wheel build issue reported above. Discussion can be found at https://github.com/pypa/pip/issues/8368

Expected time until merged

In a few days. Should be merged before #405

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

Merging #406 (b2618b3) into main (966f8dc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #406   +/-   ##
=======================================
  Coverage   94.47%   94.47%           
=======================================
  Files         103      103           
  Lines        6412     6412           
=======================================
  Hits         6058     6058           
  Misses        354      354           

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