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/ruff_python_ast/src/python_version.rs

@@ -55,9 +55,8 @@ impl PythonVersion {
         Self::PY37
     }
 
-    // TODO: change this to 314 when it is released
     pub const fn latest() -> Self {
-        Self::PY313
+        Self::PY314
     }
 
     /// The latest Python version supported in preview
@@ -94,7 +93,7 @@ impl PythonVersion {
 
 impl Default for PythonVersion {
     fn default() -> Self {
-        Self::PY39
+        Self::PY310
     }
 }