Stop testing free-threaded on 3.13

[Viicos]

Change Summary

Free-threaded on 3.13 is really slow, and has memory leaks no longer present in 3.14. I think it is not worth testing on 3.13 anymore.

Let's wait until the first 3.14 rc release before merging.

Related issue number

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1738 will improve performances by 16.51%

_{Comparing stop-t-313 (68c8eac) with main (3415428)}

Summary

⚡ 1 improvements
✅ 156 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_many_models_core_model	2.5 ms	2.2 ms	+16.51%