Update version number to v3.0.2 (final) and set release date in changelog.md

[rwgk]

Description

Boilerplate changes for v3.0.2 release.

Suggested changelog entry:

• Placeholder.

---

📚 Documentation preview 📚: https://pybind11--5985.org.readthedocs.build/

[rwgk]

EDIT: See #5986 for the fix (changes only the test)

---

1 failing in 1st attempt:

CI / 🐍 (macos-latest, 3.14t, -DCMAKE_CXX_STANDARD=20) / 🧪 (pull_request) Failing after 7m

=================================== FAILURES ===================================
_____________________________ test_async_callbacks _____________________________

    @pytest.mark.skipif(sys.platform.startswith("emscripten"), reason="Requires threads")
    def test_async_callbacks():
        # serves as state for async callback
        class Item:
            def __init__(self, value):
                self.value = value
    
        res = []
    
        # generate stateful lambda that will store result in `res`
        def gen_f():
            s = Item(3)
            return lambda j: res.append(s.value + j)
    
        # do some work async
        work = [1, 2, 3, 4]
        m.test_async_callback(gen_f(), work)
        # wait until work is done
        from time import sleep
    
        sleep(0.5)
>       assert sum(res) == sum(x + 3 for x in work)
E       assert 17 == 22
E        +  where 17 = sum([6, 7, 4])
E        +  and   22 = sum(<generator object test_async_callbacks.<locals>.<genexpr> at 0x4ed138b6b10>)

Item       = <class 'test_callbacks.test_async_callbacks.<locals>.Item'>
gen_f      = <function test_async_callbacks.<locals>.gen_f at 0x4ed149f8710>
res        = [6, 7, 4, 5]
sleep      = <built-in function sleep>
work       = [1, 2, 3, 4]

../../tests/test_callbacks.py:186: AssertionError

---

The test succeeded in the 2nd attempt:

https://github.com/pybind/pybind11/actions/runs/22073195054/job/63786808541?pr=5985