build(deps): update pulldown-cmark-to-cmark requirement from >=16, <=…

…19 to >=16, <=20

Updates the requirements on [pulldown-cmark-to-cmark](https://github.com/Byron/pulldown-cmark-to-cmark) to permit the latest version.
- [Release notes](https://github.com/Byron/pulldown-cmark-to-cmark/releases)
- [Changelog](https://github.com/Byron/pulldown-cmark-to-cmark/blob/main/CHANGELOG.md)
- [Commits](Byron/pulldown-cmark-to-cmark@v16.0.0...v20.0.0)

---
updated-dependencies:
- dependency-name: pulldown-cmark-to-cmark
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

prost-build/Cargo.toml

@@ -32,7 +32,7 @@ syn = { version = "2", features = ["full"], optional = true }
 
 # These two must be kept in sync, used for `cleanup-markdown` feature.
 pulldown-cmark = { version = "0.12", optional = true, default-features = false }
-pulldown-cmark-to-cmark = { version = ">=16, <=19", optional = true }
+pulldown-cmark-to-cmark = { version = ">=16, <=20", optional = true }
 
 [dev-dependencies]
 env_logger = { version = "0.11", default-features = false }