Add early stopping deprecation warning for v0.10.0

[awaelchli]

Now:

trainer = Trainer(early_stop_callback=True)

From v1.0, it will be removed. Instead, configure it like so:

trainer = Trainer(callbacks=[EarlyStopping(monitor=...)])

In follow up PR, will update tests with this change

[codecov]

Codecov Report

Merging #3844 into master will decrease coverage by 1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #3844    +/-   ##
=======================================
- Coverage      83%     82%    -1%     
=======================================
  Files         112     116     +4     
  Lines        8848    9823   +975     
=======================================
+ Hits         7384    8058   +674     
- Misses       1464    1765   +301