Recent legacy checkpoints are not tested in CI

[akihironitta]

🐛 Bug

Currently, only legacy checkpoints generated with versions from v1.0.0 to v1.4.1 are tested in CI, which are the versions listed in legacy/back-compatible-versions.txt.

For example, see this run on 1.6.0rc1: https://github.com/PyTorchLightning/pytorch-lightning/runs/5695770164?check_suite_focus=true#step:17:584

tests/checkpointing/test_legacy_checkpoints.py::test_load_legacy_checkpoints[1.0.0] PASSED [ 18%]
tests/checkpointing/test_legacy_checkpoints.py::test_load_legacy_checkpoints[1.0.1] PASSED [ 18%]
tests/checkpointing/test_legacy_checkpoints.py::test_load_legacy_checkpoints[1.0.2] PASSED [ 18%]
...
tests/checkpointing/test_legacy_checkpoints.py::test_load_legacy_checkpoints[1.3.8] PASSED [ 19%]
tests/checkpointing/test_legacy_checkpoints.py::test_load_legacy_checkpoints[1.4.0] PASSED [ 19%]
tests/checkpointing/test_legacy_checkpoints.py::test_load_legacy_checkpoints[1.4.1] PASSED [ 19%]
tests/checkpointing/test_legacy_checkpoints.py::test_resume_legacy_checkpoints[1.0.0] PASSED [ 19%]
tests/checkpointing/test_legacy_checkpoints.py::test_resume_legacy_checkpoints[1.0.1] PASSED [ 19%]
tests/checkpointing/test_legacy_checkpoints.py::test_resume_legacy_checkpoints[1.0.2] PASSED [ 19%]
...
tests/checkpointing/test_legacy_checkpoints.py::test_resume_legacy_checkpoints[1.3.8] PASSED [ 20%]
tests/checkpointing/test_legacy_checkpoints.py::test_resume_legacy_checkpoints[1.4.0] PASSED [ 21%]
tests/checkpointing/test_legacy_checkpoints.py::test_resume_legacy_checkpoints[1.4.1] PASSED [ 21%]

The file was introduced in #8535 and it hasn't been updated at all since then.

Expected behavior

Run tests on legacy checkpoints generated with versions from v1.0.0 to the latest version in CI.

Environment

n/a

Additional context

Found this issue while reviewing/documenting our CI/CD.

cc @tchaton @rohitgr7 @akihironitta @carmocca @Borda @awaelchli @ananthsub @ninginthecloud

[carmocca]

Duplicate of #11403