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_workspace/src/options.rs

@@ -299,8 +299,8 @@ pub struct Options {
     /// code upgrades, like rewriting type annotations. Ruff will not propose
     /// changes using features that are not available in the given version.
     ///
-    /// For example, to represent supporting Python >=3.10 or ==3.10
-    /// specify `target-version = "py310"`.
+    /// For example, to represent supporting Python >=3.11 or ==3.11
+    /// specify `target-version = "py311"`.
     ///
     /// If you're already using a `pyproject.toml` file, we recommend
     /// `project.requires-python` instead, as it's based on Python packaging
@@ -327,8 +327,8 @@ pub struct Options {
     /// file than it would for an equivalent runtime file with the same target
     /// version.
     #[option(
-        default = r#""py39""#,
-        value_type = r#""py37" | "py38" | "py39" | "py310" | "py311" | "py312" | "py313""#,
+        default = r#""py310""#,
+        value_type = r#""py37" | "py38" | "py39" | "py310" | "py311" | "py312" | "py313" | "py314""#,
         example = r#"
             # Always generate Python 3.7-compatible code.
             target-version = "py37"