Bump the libcnb group with 3 updates

Bumps the libcnb group with 3 updates: [libcnb](https://github.com/heroku/libcnb.rs), [libherokubuildpack](https://github.com/heroku/libcnb.rs) and [libcnb-test](https://github.com/heroku/libcnb.rs).


Updates `libcnb` from 0.14.0 to 0.15.0
- [Release notes](https://github.com/heroku/libcnb.rs/releases)
- [Changelog](https://github.com/heroku/libcnb.rs/blob/main/CHANGELOG.md)
- [Commits](heroku/libcnb.rs@v0.14.0...v0.15.0)

Updates `libherokubuildpack` from 0.14.0 to 0.15.0
- [Release notes](https://github.com/heroku/libcnb.rs/releases)
- [Changelog](https://github.com/heroku/libcnb.rs/blob/main/CHANGELOG.md)
- [Commits](heroku/libcnb.rs@v0.14.0...v0.15.0)

Updates `libcnb-test` from 0.14.0 to 0.15.0
- [Release notes](https://github.com/heroku/libcnb.rs/releases)
- [Changelog](https://github.com/heroku/libcnb.rs/blob/main/CHANGELOG.md)
- [Commits](heroku/libcnb.rs@v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: libcnb
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: libcnb
- dependency-name: libherokubuildpack
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: libcnb
- dependency-name: libcnb-test
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: libcnb
...

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

buildpacks/ruby/Cargo.toml

@@ -14,8 +14,8 @@ glob = "0.3"
 indoc = "2"
 # libcnb has a much bigger impact on buildpack behaviour than any other dependencies,
 # so it's pinned to an exact version to isolate it from lockfile refreshes.
-libcnb = "=0.14.0"
-libherokubuildpack = "=0.14.0"
+libcnb = "=0.15.0"
+libherokubuildpack = "=0.15.0"
 rand = "0.8"
 regex = "1"
 serde = "1"
@@ -27,5 +27,5 @@ url = "2"
 clap = { version = "4", features = ["derive"] }
 
 [dev-dependencies]
-libcnb-test = "=0.14.0"
+libcnb-test = "=0.15.0"
 toml = "0.8"

commons/Cargo.toml

@@ -14,8 +14,8 @@ indoc = "2"
 lazy_static = "1"
 # libcnb has a much bigger impact on buildpack behaviour than any other dependencies,
 # so it's pinned to an exact version to isolate it from lockfile refreshes.
-libcnb = "=0.14.0"
-libherokubuildpack = "=0.14.0"
+libcnb = "=0.15.0"
+libherokubuildpack = "=0.15.0"
 regex = "1"
 serde = "1"
 sha2 = "0.10"