Fixes an elided lifetime causing a warning with newer compiler versions

[xanathar]

Super minor change, fixes a warning with latest compiler versions:

warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/lib.rs:555:32
    |
555 | pub fn parse_challenges(input: &str) -> Result<Vec<ChallengeRef>, parser::Error> {
    |                                ^^^^                ------------   ------------- the same lifetime is hidden here
    |                                |                   |
    |                                |                   the same lifetime is hidden here
    |                                the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
555 | pub fn parse_challenges(input: &str) -> Result<Vec<ChallengeRef<'_>>, parser::Error<'_>> {
    |                                                                ++++                ++++

[scottlamb]

Thank you!

CI was failing to run on this branch due to a couple other problems: GitHub Actions rot, and a similar error in the fuzz crate. I pushed your commit and a couple others to fix.