Cargo.toml: update clap version to 3

Version 2 is really old. We can update to 3 very easily. But it seems 4 will
require some changes, so let's do that in a separate step. This is what
https://github.com/clap-rs/clap/blob/v4.5.21/CHANGELOG.md#migrating recommends.

Author: keszybz

Cargo.toml

@@ -14,7 +14,7 @@ exclude = ["tests/07a-mount-point-excl", "tests/10-example"]
 
 [dependencies]
 anyhow = "1.0.12"
-clap = { version = "2.33", default-features = false }
+clap = { version = "3", default-features = false, features = ["std", "cargo"] }
 liboverdrop = "0.1.0"
 rust-ini = ">=0.15, <0.18"
 log = { version = "0.4", features = ["std"] }