Bump bigdecimal from 3.1.8 to 3.1.9 in /spec/tester_mongo

Bumps [bigdecimal](https://github.com/ruby/bigdecimal) from 3.1.8 to 3.1.9.
- [Release notes](https://github.com/ruby/bigdecimal/releases)
- [Changelog](https://github.com/ruby/bigdecimal/blob/master/CHANGES.md)
- [Commits](ruby/bigdecimal@v3.1.8...v3.1.9)

---
updated-dependencies:
- dependency-name: bigdecimal
  dependency-type: indirect
  update-type: version-update:semver-patch
...

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

spec/tester_mongo/Gemfile.lock

@@ -88,7 +88,7 @@ GEM
     base64 (0.2.0)
     bcrypt (3.1.20)
     benchmark (0.4.0)
-    bigdecimal (3.1.8)
+    bigdecimal (3.1.9)
     bootsnap (1.18.4)
       msgpack (~> 1.2)
     bson (5.0.2)