client/cli/src/config: Warn on low file descriptor limit (#6956)

* client/cli/src/config: Warn on low file descriptor limit

Substrate sets the soft file descriptor limit to the hard limit at
startup. In the case of the latter being low already (< 10_000) a
Substrate node under high demand might run into issues e.g. when opening
up new TCP connections or persisting data to the database.

With this commit a warn message is printed to stderr.

* client/cli/Cargo.toml: Update to fdlimit 0.2.0