Update which requirement from 6.0.0 to 7.0.0

Updates the requirements on [which](https://github.com/harryfei/which-rs) to permit the latest version.
- [Release notes](https://github.com/harryfei/which-rs/releases)
- [Changelog](https://github.com/harryfei/which-rs/blob/master/CHANGELOG.md)
- [Commits](harryfei/which-rs@6.0.0...7.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

capi/Cargo.toml

@@ -51,7 +51,7 @@ harness = false
 
 [build-dependencies]
 cbindgen = {version = "0.27", optional = true}
-which = {version = "6.0.0", optional = true}
+which = {version = "7.0.0", optional = true}
 
 [dependencies]
 # Pinned, because we use #[doc(hidden)] APIs.