deps: upgrade to libuv 1.21.0

Notable changes:

- Building via cmake is now supported.
  PR-URL: https://github.com/libuv/libuv/pull/1850
- Stricter checks have been added to prevent watching the same
  file descriptor multiple times.
  PR-URL: https://github.com/libuv/libuv/pull/1851
  Refs: https://github.com/nodejs/node/issues/3604
- An IPC deadlock on Windows has been fixed.
  PR-URL: https://github.com/libuv/libuv/pull/1843
  Fixes: https://github.com/nodejs/node/issues/9706
  Fixes: https://github.com/nodejs/node/issues/7657
- uv_fs_lchown() has been added.
  PR-URL: https://github.com/libuv/libuv/pull/1826
  Refs: https://github.com/nodejs/node/issues/19868
- uv_fs_copyfile() sets errno on error.
  PR-URL: https://github.com/libuv/libuv/pull/1881
  Fixes: https://github.com/nodejs/node/issues/21329
- uv_fs_fchmod() supports -A files on Windows.
  PR-URL: https://github.com/libuv/libuv/pull/1819
  Refs: https://github.com/nodejs/node/issues/12803

PR-URL: https://github.com/nodejs/node/pull/21466


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>