gnu: icedove: Use updated clang and llvm.

* gnu/packages/gnuzilla.scm (icedove)[native-inputs]: Replace `clang-11`
and `clang-11` with the default version, which is now 13.

gnu/packages/gnuzilla.scm

@@ -1271,8 +1271,8 @@ standards of the IceCat project.")
               (base32
                "0i2wpi08jfksqqc0j6q362nyk3zmlqd5ds2w7mm74infvrw6fzzq")))))
        ("cargo" ,rust "cargo")
-       ("clang" ,clang-11)
-       ("llvm" ,llvm-11)
+       ("clang" ,clang)
+       ("llvm" ,llvm)
        ("m4" ,m4)
        ("nasm" ,nasm)
        ("node" ,node)