Allow bootsnap to be disabled

What does this MR do and why?

While diagnosing gitlab-development-kit!2236 (closed), I discovered there was no way to properly disable Bootsnap for gitlab-org/gitlab. This MR fixes that by allowing ENABLE_BOOTSNAP to be set to 0, no or false.

In this MR I also upgraded bootsnap from 1.4.6 to 1.9.1, added require: false given we only 'load' bootsnap if:

require 'bootsnap/setup' if ENV['RAILS_ENV'] != 'production' && !%w(0 no false).include?(ENV['ENABLE_BOOTSNAP'])

Related: gitlab-development-kit!2239 (merged)

How to set up and validate locally

1. In your GDK_ROOT/gitlab directory, check out the ashmckenzie/allow-bootsnap-to-be-disabled branch for GitLab:

   git fetch && git checkout ashmckenzie/allow-bootsnap-to-be-disabled

2. In your GDK_ROOT directory, ensure you are up-to-date:

   git fetch && git checkout main && git pull -r

3. Disable gitlab.rails.bootsnap:

   gdk config set gitlab.rails.bootsnap false

4. Reconfigure your GDK:

   gdk reconfigure

5. Stop your GDK:

   gdk stop && gdk kill

6. Remove any existing bootsnap cache:

   rm -rf gitlab/tmp/cache/bootsnap-*

7. Start your GDK:

   gdk start

8. Wait until Rails has fully loaded (check http://localhost:3000)

9. Confirm that bootsnap has not recreated any cache:

   ls -la gitlab/tmp/cache/bootsnap-*

It's also worth testing with a clean tmp/cache and gdk config set gitlab.rails.bootsnap true to verify that cache is still created by default or when gitlab.rails.bootsnap is explicitly set.

🎗 Once tested, don't forget to run gdk config set gitlab.rails.bootsnap true && gdk reconfigure if you want the default behaviour to return.

MR acceptance checklist

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