Update default and latest Python versions for 3.14 (#20725)

Summary
--

Closes #19467 and also removes the warning about using Python 3.14
without
preview enabled.

I also bumped `PythonVersion::default` to 3.9 because it reaches EOL
this month,
but we could also defer that for now if we wanted.

The first three commits are related to the `latest` bump to 3.14; the
fourth commit
bumps the default to 3.10.

Note that this PR also bumps the default Python version for ty to 3.10
because
there was a test asserting that it stays in sync with
`ast::PythonVersion`.

Test Plan
--

Existing tests

I spot-checked the ecosystem report, and I believe these are all
expected. Inbits doesn't specify a target Python version, so I guess
we're applying the default. UP007, UP035, and UP045 all use the new
default value to emit new diagnostics.

crates/ty_python_semantic/resources/mdtest/binary/classes.md

@@ -18,6 +18,11 @@ reveal_type(A | B)  # revealed: UnionType
 
 ## Union of two classes (prior to 3.10)
 
+```toml
+[environment]
+python-version = "3.9"
+```
+
 ```py
 class A: ...
 class B: ...