ci: move more ci onto Python 3.14

[davidhewitt]

This updates a bunch of CI jobs to use Python 3.14 (including the benchmarks).

[codspeed-hq]

CodSpeed Performance Report

Merging #5660 will degrade performances by 14.39%

_{Comparing davidhewitt:more-ci-3.14 (038127b) with main (1e00af5)}

Summary

⚡ 2 improvements
❌ 6 regressions
✅ 90 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	err_new_restore_and_fetch	7.7 µs	7 µs	+10.07%
❌	iter_list	5.7 ms	6.6 ms	-12.31%
❌	list_nth	3.8 µs	4.4 µs	-13.03%
❌	list_nth_back	3.9 µs	4.5 µs	-12.59%
⚡	drop_many_objects	3.7 µs	3.2 µs	+16.44%
❌	test_method_call	14.8 µs	16.7 µs	-11.1%
❌	test_many_keyword_arguments_py	16.9 µs	19.7 µs	-14.39%
❌	test_many_keyword_arguments_rs	18.7 µs	21.3 µs	-11.91%