Bump omnibus from 15122f2 to 9c0643a in /omnibus (#70)

Bumps [omnibus](https://github.com/chef/omnibus) from `15122f2` to `9c0643a`.
- [Release notes](https://github.com/chef/omnibus/releases)
- [Commits](15122f2565...9c0643a3a4)

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

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

omnibus/Gemfile.lock

@@ -8,10 +8,10 @@ GIT
 
 GIT
   remote: https://github.com/chef/omnibus.git
-  revision: 15122f2565b4d80f81d4658dfabdf751aaa990ca
+  revision: 9c0643a3a44f3e7119789c2093bfc8edd78c74ff
   branch: main
   specs:
-    omnibus (9.0.20)
+    omnibus (9.0.21)
       aws-sdk-s3 (~> 1.116.0)
       chef-cleanroom (~> 1.0)
       chef-utils (>= 15.4)
@@ -416,7 +416,7 @@ GEM
     toml-rb (2.2.0)
       citrus (~> 3.0, > 3.0)
     tomlrb (1.3.0)
-    train-core (3.10.7)
+    train-core (3.10.8)
       addressable (~> 2.5)
       ffi (!= 1.13.0)
       json (>= 1.8, < 3.0)