add: deprecate --file

[dberenbaum]

Part of #7093

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (7863771) 91.03% compared to head (f0a2fa8) 91.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9547      +/-   ##
==========================================
- Coverage   91.03%   91.02%   -0.02%     
==========================================
  Files         472      472              
  Lines       36449    36430      -19     
  Branches     5225     5225              
==========================================
- Hits        33181    33160      -21     
- Misses       2701     2702       +1     
- Partials      567      568       +1     

Impacted Files	Coverage Δ
dvc/commands/add.py	90.32% <ø> (-0.31%)	⬇️
tests/func/test_add.py	98.95% <ø> (-0.03%)	⬇️
tests/unit/command/test_add.py	100.00% <ø> (ø)

... and 5 files with indirect coverage changes

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[skshetry]

@dberenbaum, do you mind removing this line as well?

dvc/dvc/repo/add.py

Lines 54 to 55 in 2c43655

	if len(targets) > 1 and kwargs.get("file"):
	raise InvalidArgumentError("cannot use '--file' with multiple targets")

EDIT: created #9548.