Bump transitive rustls-webpki dependency

[T0mstone]

...so that I can stop getting the annoying dependabot alert E-mails.
(I also considered changing my notif settings, but since there's apparently a vulnerability, why not stop people from running vulnerable code when running the tests that fetch something from the web)

We can then later remove the extra dependency line again after ureq updates its rustls dependency and we update to that newer ureq version.

[MDLC01]

I don't understand why we need rustls-webpki even though it's not used in the code

[T0mstone]

It's a transitive dependency (ureq depends on rustls, which depends on rustls-webpki). Adding an it as an explicit dependency tells Cargo's version resolver to use that version as the minimum, which is higher than what it would otherwise have chosen.