title: Heroku push error: "Your bundle is locked to mimemagic (0.3.5) from rubygems repository" AFTER update to Rails 6.1.3.2
body: <p>I keep receiving this error <code>Your bundle is locked to mimemagic (0.3.5) from rubygems repository</code> when pushing to heroku.</p>
  <p>I read the <a href="https://github.com/rails/rails/issues/41750" rel="nofollow noreferrer">resolved issue on Github</a> where the Rails team swapped the dependency on ActiveStorage from mimemagic to mini_mime. So I did what was recommended and upgraded my app from Rails 6.0.3 to 6.1.3.2. My Gemfile now has <code>gem 'rails', '&gt;=6.1.3.2'</code>.</p>
  <p>I ran <code>bundle update</code> which updated all the gems and I did a <code>bundle uninstall mimemagic</code> for good measure.</p>
  <p>I verified in my Gemfile.lock that there is no longer a reference to mime_magic. Instead, I can see that activestorage has the updated dependency on mini_mime:</p>
  <pre class="lang-rb prettyprint-override"><code>activestorage (6.1.3.2)
    actionpack (= 6.1.3.2)
    activejob (= 6.1.3.2)
    activerecord (= 6.1.3.2)
    activesupport (= 6.1.3.2)
    marcel (~&gt; 1.0.0)
    mini_mime (~&gt; 1.0.2)
  </code></pre>
  <p>All changes are committed in the repo. However, when I deploy to Heroku via <code>git push heroku main</code>, I still receive this <code>Your bundle is locked to mimemagic (0.3.5) from rubygems repository</code> error. How can there be a dependency when all gems were updated and no longer reference it, and its no longer present in both my local machine or Gemfile.lock?</p>