[stdlib] Allow loop.create_task eager_start on Python 3.13

[nightcityblade]

Fixes#15269.

Python 3.13.3 added arbitrary keyword forwarding to event-loop create_task, which allows eager_start to be passed to a compatible task factory. Move the existing loop-specific eager_start signature gate from Python 3.14 to Python 3.13 while leaving the high-level asyncio.create_task and TaskGroup.create_task gates unchanged.

Add regression coverage for both AbstractEventLoop and BaseEventLoop.

Tests:

• pre-commit run --files stdlib/asyncio/base_events.pyi stdlib/asyncio/events.pyi stdlib/@tests/test_cases/asyncio/check_task.py
• python tests/mypy_test.py stdlib/asyncio -p 3.13
• python tests/pyright_test.py stdlib/asyncio --pythonversion 3.13 -p pyrightconfig.stricter.json
• python tests/pyright_test.py stdlib/@tests/test_cases/asyncio/check_task.py --pythonversion 3.13 -p pyrightconfig.testcases.json
• python tests/regr_test.py stdlib -p 3.13

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉