Update deny config

[djc]

cargo-deny got updated and changed some configuration stuff:

djc-2021 main webpki $ cargo deny check licenses
error[deprecated]: this key has been removed, see https://github.com/EmbarkStudios/cargo-deny/pull/611 for migration information
  ┌─ /Users/djc/src/webpki/deny.toml:2:1
  │
2 │ unmaintained = "deny"
  │ ━━━━━━━━━━━━

error[deprecated]: this key has been removed, see https://github.com/EmbarkStudios/cargo-deny/pull/611 for migration information
  ┌─ /Users/djc/src/webpki/deny.toml:4:1
  │
4 │ notice = "deny"
  │ ━━━━━━

2024-08-03 15:03:31 [ERROR] failed to validate configuration file /Users/djc/src/webpki/deny.toml

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.30%. Comparing base (1414bf4) to head (2e1f2f5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #277 +/- ## ======================================= Coverage 97.30% 97.30% ======================================= Files 19 19 Lines 4237 4237 ======================================= Hits 4123 4123 Misses 114 114 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[djc]

Didn't realise we had cargo-deny going here -- I guess it's a carry over from the original version.

Yup, it was first added by Brian in December of 2020.

Note the "We don't maintain a fixed Cargo.lock so enforcing ..." comment near the bottom of this file is now obsolete.

Removed the comment and flipped the setting to multipe-versions = "false".