Adding test for legacy checkpoint created with 2.6.0

[pl-ghost]

This is automated addition of created checkpoint with the latest 2.6.0 lightning release!

cc @lantiga

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu-guardian	success	✅

These checks are required after the changes to tests/legacy/back-compatible-versions.txt.

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Thank you for your contribution! 💜

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (8540370) to head (723934a).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (8540370) and HEAD (723934a). Click for more details.

HEAD has 385 uploads less than BASE

Flag	BASE (8540370)	HEAD (723934a)
cpu	119	30
python3.12.7	35	9
lightning_fabric	30	0
pytest	60	0
python3.12	36	9
python3.10	12	3
lightning	59	15
python3.11	24	6
python	12	3
pytorch2.1	12	6
pytest-full	59	30
pytorch2.5.1	6	3
pytorch_lightning	30	15
pytorch2.6	6	3
pytorch2.2.2	6	3
pytorch2.3	6	3
pytorch2.8	6	3
pytorch2.7	6	3
pytorch2.4.1	5	3
pytorch2.9	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21388     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         269      266      -3     
  Lines       23804    23749     -55     
=========================================
- Hits        20626    18725   -1901     
- Misses       3178     5024   +1846