Update rustc in flake.nix to 1.71.0.

ruff has updated their minimum rust version, and due to
#15939, we have to
follow along.

flake.nix

@@ -82,7 +82,7 @@
                   #
                   # NOTE: We currently need to set the Rust version unnecessarily high
                   # in order to work around https://github.com/matrix-org/synapse/issues/15939
-                  (rust-bin.stable."1.70.0".default.override {
+                  (rust-bin.stable."1.71.0".default.override {
                     # Additionally install the "rust-src" extension to allow diving into the
                     # Rust source code in an IDE (rust-analyzer will also make use of it).
                     extensions = [ "rust-src" ];