meson: Update our rust dependencies

These are the package versions currently shipped by Fedora. This allows
using system packages by setting

    export MESON_PACKAGE_CACHE_DIR=/usr/share/cargo/registry/

Of course, other distros may place it somewhere else.

Ubuntu matches versions on syn and unicode-ident but is a tiny bit off
on quote and proc-macro2. However, given how far I was able to bump the
versions with only a tiny meson tweak to syn, I think it should work
with the Ubuntu versions as well.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26726>

subprojects/packagefiles/proc-macro2/meson.build

@@ -48,7 +48,7 @@ lib = static_library(
   'proc_macro2',
   'src/lib.rs',
   rust_args : rust_args,
-  override_options : ['rust_std=2018', 'build.rust_std=2018'],
+  override_options : ['rust_std=2021', 'build.rust_std=2021'],
   link_with : u_ind,
   rust_abi : 'rust',
   native : true,