fix(authenticator): u2f cannot handle the chrono 0.4.30 breaking change

[lucasfernog]

see https://github.com/chronotope/chrono/releases/tag/v0.4.30

Compiling u2f v0.2.0
error[E0308]: mismatched types
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/u2f-0.2.0/src/util.rs:25:5
   |
20 | pub fn expiration(timestamp: String) -> Duration {
   |                                         -------- expected `time::Duration` because of return type
...
25 |     now.signed_duration_since(ts.unwrap())
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `time::Duration`, found `chrono::Duration`
   |
   = note: `chrono::Duration` and `time::Duration` have similar names, but are actually distinct types
note: `chrono::Duration` is defined in crate `chrono`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/chrono-0.4.31/src/duration.rs:55:1
   |
55 | pub struct Duration {
   | ^^^^^^^^^^^^^^^^^^^
note: `time::Duration` is defined in crate `time`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/time-0.1.45/src/duration.rs:45:1
   |
45 | pub struct Duration {
   | ^^^^^^^^^^^^^^^^^^^