build(deps): bump rack from 3.1.15 to 3.1.16 in /appengine/pubsub (#1574)

Bumps [rack](https://github.com/rack/rack) from 3.1.15 to 3.1.16.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.1.15...v3.1.16)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.1.16
  dependency-type: indirect
...

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

Author: dependabot[bot]

appengine/pubsub/Gemfile.lock

@@ -69,7 +69,7 @@ GEM
       uri
     os (1.1.4)
     public_suffix (5.0.4)
-    rack (3.1.15)
+    rack (3.1.16)
     rack-protection (4.0.0)
       base64 (>= 0.1.0)
       rack (>= 3.0.0, < 4)