Bump bootsnap from 1.9.3 to 1.9.4 in /backend

Bumps [bootsnap](https://github.com/Shopify/bootsnap) from 1.9.3 to 1.9.4.
- [Release notes](https://github.com/Shopify/bootsnap/releases)
- [Changelog](https://github.com/Shopify/bootsnap/blob/master/CHANGELOG.md)
- [Commits](Shopify/bootsnap@v1.9.3...v1.9.4)

---
updated-dependencies:
- dependency-name: bootsnap
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

backend/Gemfile.lock

@@ -63,7 +63,7 @@ GEM
     addressable (2.8.0)
       public_suffix (>= 2.0.2, < 5.0)
     bindex (0.8.1)
-    bootsnap (1.9.3)
+    bootsnap (1.9.4)
       msgpack (~> 1.0)
     builder (3.2.4)
     byebug (11.1.3)