ci: test against jupyterhub 5 and python 3.13, stop testing jupyterhub 2 and python 3.8

[consideRatio]

I'm seeing test failures in tljh for jupyterhub 5.2.0, but not for 5.1.0, for when hubtraf User object's method ensure_server_simulate is called.

From this test run on GitHub Actions

    async def test_user_code_execute():
        """
        User logs in, starts a server & executes code
        """
        username = secrets.token_hex(8)

        assert (
            0
            == await (
                await asyncio.create_subprocess_exec(
                    *TLJH_CONFIG_PATH, "set", "auth.type", "dummy"
                )
            ).wait()
        )
        assert (
            0
            == await (
                await asyncio.create_subprocess_exec(*TLJH_CONFIG_PATH, "reload")
            ).wait()
        )

        async with User(username, HUB_URL, partial(login_dummy, password="")) as u:
            assert await u.login()
>           assert await u.ensure_server_simulate(timeout=60, spawn_refresh_time=5)
E           assert False

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 38.08%. Comparing base (df19256) to head (f56113a). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #49 +/- ## ======================================= Coverage 38.08% 38.08% ======================================= Files 6 6 Lines 365 365 ======================================= Hits 139 139 Misses 226 226 ```

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[yuvipanda]

ty @consideRatio