Group phf and phf_codegen updates (#29369)

## Summary

Ruff uses `phf_codegen` to generate a Unicode alias map and `phf` to
look up values in it. The generated map must be compatible with the
lookup implementation; updating `phf` alone can make lookups fail. For
example, [#29362](https://github.com/astral-sh/ruff/pull/29362) fails
Unicode alias tests.

Group Renovate updates to both crates when both are available so they
can be reviewed together, following the existing rule for `strum` and
`strum_macros`. Renovate can still propose an update to one crate on its
own if the other update is unavailable.

.github/renovate.json5

@@ -60,6 +60,12 @@
       matchPackageNames: ["monaco"],
       description: "Weekly update of the Monaco editor",
     },
+    {
+      groupName: "phf",
+      matchManagers: ["cargo"],
+      matchPackageNames: ["phf", "phf_codegen"],
+      description: "Weekly update of phf dependencies",
+    },
     {
       groupName: "strum",
       matchManagers: ["cargo"],