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/stubs/ellipsis.md

@@ -47,7 +47,7 @@ Iterating over an ellipsis literal as part of a `for` loop in a stub is invalid,
 results in a diagnostic:
 
 ```pyi
-# error: [not-iterable] "Object of type `ellipsis` is not iterable"
+# error: [not-iterable] "Object of type `EllipsisType` is not iterable"
 for a, b in ...:
     reveal_type(a)  # revealed: Unknown
     reveal_type(b)  # revealed: Unknown
@@ -59,13 +59,13 @@ In a non-stub file, there's no special treatment of ellipsis literals. An ellips
 be assigned if `EllipsisType` is actually assignable to the annotated type.
 
 ```py
-# error: 7 [invalid-parameter-default] "Default value of type `ellipsis` is not assignable to annotated parameter type `int`"
+# error: 7 [invalid-parameter-default] "Default value of type `EllipsisType` is not assignable to annotated parameter type `int`"
 def f(x: int = ...) -> None: ...
 
-# error: 1 [invalid-assignment] "Object of type `ellipsis` is not assignable to `int`"
+# error: 1 [invalid-assignment] "Object of type `EllipsisType` is not assignable to `int`"
 a: int = ...
 b = ...
-reveal_type(b)  # revealed: ellipsis
+reveal_type(b)  # revealed: EllipsisType
 ```
 
 ## Use of `Ellipsis` symbol
@@ -73,6 +73,6 @@ reveal_type(b)  # revealed: ellipsis
 There is no special treatment of the builtin name `Ellipsis` in stubs, only of `...` literals.
 
 ```pyi
-# error: 7 [invalid-parameter-default] "Default value of type `ellipsis` is not assignable to annotated parameter type `int`"
+# error: 7 [invalid-parameter-default] "Default value of type `EllipsisType` is not assignable to annotated parameter type `int`"
 def f(x: int = Ellipsis) -> None: ...
 ```