models/default_versions: Remove sync fns

[Turbo87]

This PR first migrates the remaining code paths to use the async variants of the fns in the models/default_versions module, and then removes the now-unused sync functions, while also renaming the async variants and dropping the async_ prefix.

• ... as promised in models/default_versions: Add async fn variants #10036

[codecov]

Codecov Report

Attention: Patch coverage is 37.28814% with 37 lines in your changes missing coverage. Please review.

Project coverage is 88.94%. Comparing base (df815d7) to head (c888aff).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/crates-admin/delete_version.rs	0.00%	35 Missing ⚠️
src/bin/crates-admin/default_versions.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10047      +/-   ##
==========================================
- Coverage   89.44%   88.94%   -0.51%     
==========================================
  Files         295      295              
  Lines       31475    31462      -13     
==========================================
- Hits        28154    27985     -169     
- Misses       3321     3477     +156     

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