Rename internal serialization function

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.61%. Comparing base (223f19d) to head (b093373). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
foamlib/_files/_serialization.py	84.61%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #240 +/- ## ======================================= Coverage 88.61% 88.61% ======================================= Files 16 16 Lines 1239 1239 ======================================= Hits 1098 1098 Misses 141 141 ```

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