Add version suffix to last.ckpt
🚀 Feature
If you use ModelCheckpoint(save_last=True) and you run an experiment twice in the same directory, then this set of checkpoints is generated:
file-v0.ckpt
file-v1.ckpt
...
last.ckpt (the last of the second run)

the idea is to add a version also to last.ckpt if it would get overwritten:
file-v0.ckpt
file-v1.ckpt
...
last-v0.ckpt (last of the first run)
last-v1.ckpt (last of the second run)

Motivation
Avoid overwriting existing checkpoints
Alternatives
Modifying the default CHECKPOINT_NAME_LAST to avoid the conflict
Additional context
Discussed in #5000 (comment)
To be done after #5008 is merged.
cc @Borda @carmocca @awaelchli @ninginthecloud @jjenniferdai @rohitgr7