Adhere to unchecked-duration-subtraction lint rename

Recent versions of Rust have renamed the unchecked-duration-subtraction
lint to unchecked-time-subtraction. Adhere to this rename to silence any
warnings pertaining the usage of the former.

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

Cargo.toml

@@ -46,7 +46,7 @@ doc-markdown = "warn"
 join-absolute-paths = "warn"
 large-enum-variant = "warn"
 redundant-closure-for-method-calls = "warn"
-unchecked-duration-subtraction = "warn"
+unchecked-time-subtraction = "warn"
 uninlined-format-args = "warn"
 use-self = "warn"
 wildcard-imports = "warn"