fix internal call to deprecated train_loop

[awaelchli]

What does this PR do?

On master, a deprecation message is shown due to an internal call in ModelCheckpoint to trainer.train_loop, which was renamed to fit_loop.

LightningDeprecationWarning: `Trainer.train_loop` has been renamed to `Trainer.fit_loop` and will be removed in v1.6.
  rank_zero_deprecation(

This issue is not present in any releases, no changelog needed.

Does your PR introduce any breaking changes ? If yes, please list them.

no

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[codecov]

Codecov Report

Merging #8434 (17f2ae5) into master (176df20) will decrease coverage by 5%.
The diff coverage is 100%.

❗ Current head 17f2ae5 differs from pull request most recent head 4d77e6d. Consider uploading reports for the commit 4d77e6d to get more accurate results

@@           Coverage Diff           @@
##           master   #8434    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
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  Files         216     216            
  Lines       14109   14109            
=======================================
- Hits        13083   12421   -662     
- Misses       1026    1688   +662