Refresh git cheat sheet

Why is this change being made?

The Git cheat sheet was messy and a bit outdated, it was refreshed as part of https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/4113. This MR updates the PDF file with a new version and deletes a background image that wasn't used anymore.

This file is the 3rd result on Google when searching for git cheat sheet (behind Atlassian and GitHub):

https://www.google.com/search?q=git+cheat+sheet&oq=git+cheat+sheet&aqs=chrome..69i57j69i64l2j69i60l3.5627j0j7&sourceid=chrome&ie=UTF-8

Pages using this file

• https://about.gitlab.com/handbook/customer-success/professional-services-engineering/education-services/gitbasicshandsonlab2.html
• https://about.gitlab.com/handbook/customer-success/professional-services-engineering/education-services/fg-gitlabwithgitbasics.html
• https://about.gitlab.com/handbook/customer-success/professional-services-engineering/education-services/partner-gitlab-basics-fg.html
• https://about.gitlab.com/handbook/marketing/blog/git-guide/
• https://about.gitlab.com/handbook/marketing/brand-and-product-marketing/content/content-marketing/

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

Closes https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/4113

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