stage.save: Remove merge_versioned option.

[daavoo]

If we want add, commit and move to work for cloud versioning, the option must be always True so it is the same as not having it.

Closes #8828
(We were not passing merge_versioned=True in dvc commit)

---

After the first push:

$ mkdir data
$ echo bar > data/bar
$ echo foo > data/foo
$ dvc add data
$ dvc push -v

Tracking local updates:

$ echo bar2 > data/bar

Works as expected with either dvc add data or dvc commit:

$ dvc add data
$ cat data.dvc
outs:
- path: data
  files:
  - size: 5
    md5: 042967ff088f7f436015fbebdcad1f28
    relpath: bar
  - size: 4
    version_id: Vnu6l9_ECdLGrp6B5iK6YmQTm87UaDzP
    etag: d3b07384d113edec49eaa6238ad5ff00
    md5: d3b07384d113edec49eaa6238ad5ff00
    relpath: foo

$ dvc commit
$ cat data.dvc
outs:
- path: data
  files:
  - size: 5
    md5: 042967ff088f7f436015fbebdcad1f28
    relpath: bar
  - size: 4
    version_id: Vnu6l9_ECdLGrp6B5iK6YmQTm87UaDzP
    etag: d3b07384d113edec49eaa6238ad5ff00
    md5: d3b07384d113edec49eaa6238ad5ff00
    relpath: foo

[codecov]

Codecov Report

Base: 93.72% // Head: 93.72% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (e7451ea) compared to base (8bda365).
Patch coverage: 95.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8873   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files         453      453           
  Lines       36146    36154    +8     
  Branches     5239     5238    -1     
=======================================
+ Hits        33877    33885    +8     
  Misses       1761     1761           
  Partials      508      508           

Impacted Files	Coverage Δ
dvc/repo/worktree.py	10.25% <0.00%> (ø)
dvc/repo/add.py	100.00% <100.00%> (ø)
dvc/stage/__init__.py	94.69% <100.00%> (-0.02%)	⬇️
tests/func/test_commit.py	100.00% <100.00%> (ø)

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[dberenbaum]

I will leave QA up to you and reviewers, but LGTM otherwise

[pmrowla]

linking my comment from the old PR for future reference

We can probably just remove the flag then. This issue is more a side effect of our current add/commit/move behavior, where we always create a new stage from scratch, and then load the previous version if it exists later and try to merge the two, (as opposed to loading the stage if it exists first and then modifying it)

Originally posted by @pmrowla in #8829 (comment)