gc: fix usage with --date

[dtrifiro]

• gc --date was broken because the wrong attribute was being accessed resulting in an unexpected exception.
• --date was being ignored by brancher unless one of --all-branches, --all-tags, --all-commits, --all-experiments was also provided.
• added test for --date

Originally reported on discord

[codecov]

Codecov Report

Base: 93.39% // Head: 93.40% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c494c79) compared to base (db70d1c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8880   +/-   ##
=======================================
  Coverage   93.39%   93.40%           
=======================================
  Files         453      453           
  Lines       36345    36359   +14     
  Branches     5268     5267    -1     
=======================================
+ Hits        33946    33960   +14     
  Misses       1885     1885           
  Partials      514      514           

Impacted Files	Coverage Δ
dvc/repo/brancher.py	100.00% <100.00%> (ø)
dvc/scm.py	86.42% <100.00%> (-0.20%)	⬇️
tests/func/test_gc.py	100.00% <100.00%> (ø)
tests/unit/scm/test_scm.py	100.00% <100.00%> (ø)

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