Updated statsmodels breaking changes 0.15.0

[jmafoster1]

Summary

This PR updates the CTF to statsmodels 0.15.0, which introduced a breaking change from 0.14.6 :(.

Main file changes

linear_regression_estimator.py - removed cols=None argument from model.conf_int.
PULL_REQUEST_TEMPLATE.md - added Summary and Main file changes headings.

Checklist

Before you mark your PR as ready for review, please ensure you have completed the following.

• Added unit tests for new functionality
• Properly documented any new code
• Updated Sphinx documentation in docs/ and checked for updated configuration/runtime options
• Added a jupyter notebook tutorial in docs/source/tutorials for any new functionality
• Verified local docs build with sphinx-build -W source/ build

[github-actions]

🦙 MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
⚠️ PYTHON	black	30		1	1.26s
✅ PYTHON	pylint	30		0	6.26s

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.48%. Comparing base (189396e) to head (f79d7f3).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #407   +/-   ##
=======================================
  Coverage   98.48%   98.48%           
=======================================
  Files          25       25           
  Lines        1517     1517           
=======================================
  Hits         1494     1494           
  Misses         23       23           

Files with missing lines	Coverage Δ
..._testing/estimation/linear_regression_estimator.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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