Draft: Introduce Gitlab.next_rails? to run Rails 7.1 tests

What does this MR do and why?

Similar to Draft: Introduce Gitlab.next_rails? to run Rail... (!160895 - closed), but this MR specifies BUNDLE_GEMFILE: "Gemfile.next" to install gems for Rails 7.1 version.

.gitlab/ci/preflight.gitlab-ci.yml

@@ -11,6 +11,7 @@
     - .qa-cache
   variables:
     USE_BUNDLE_INSTALL: "false"
+    BUNDLE_GEMFILE: 'Gemfile'
     SETUP_DB: "false"
   before_script:
     - !reference [.default-before_script, before_script]