Adding test for legacy checkpiont created with 1.8.0 (#15450)

* [create-pull-request] automated change

* Add legacy to checkgroup

Co-authored-by: akihironitta <akihironitta@users.noreply.github.com>
Co-authored-by: Akihiro Nitta <nitta@akihironitta.com>

.github/checkgroup.yml

@@ -21,6 +21,7 @@ subprojects:
       - "requirements/pytorch/**"
       - "src/pytorch_lightning/**"
       - "tests/tests_pytorch/**"
+      - "tests/legacy/back-compatible-versions.txt"
       - "setup.cfg"  # includes pytest config
       - ".actions/**"
     checks:

tests/legacy/back-compatible-versions.txt

@@ -70,3 +70,4 @@
 1.7.5
 1.7.6
 1.7.7
+1.8.0