build(deps): update rcgen requirement from 0.11.1 to 0.12.0 (#718)

Updates the requirements on [rcgen](https://github.com/rustls/rcgen) to permit the latest version.
- [Commits](rustls/rcgen@v0.11.1...v0.12.0)

---
updated-dependencies:
- dependency-name: rcgen
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

poem/Cargo.toml

@@ -151,7 +151,7 @@ unic-langid = { version = "0.9.0", optional = true, features = ["macros"] }
 intl-memoizer = { version = "0.5.1", optional = true }
 ring = { version = "0.16.20", optional = true }
 reqwest = { workspace = true, features = ["json"], optional = true }
-rcgen = { version = "0.11.1", optional = true }
+rcgen = { version = "0.12.0", optional = true }
 x509-parser = { version = "0.15.0", optional = true }
 tokio-metrics = { version = "0.3.0", optional = true }
 rust-embed = { version = "8.0", optional = true }