Docs: Remove trailing spaces from YAML files

Why is this change being made?

We cannot turn on the trailing spaces test in yamllint until existing files with trailing spaces are cleaned up. This merge request tackles files starting with A-H in the data directory:

	modified:   alumni.yml
	modified:   applications.yml
	modified:   blog_merch.yml
	modified:   categories.yml
	modified:   contribute_page.yml
	modified:   country_hiring_status.yml
	modified:   developer_report_2019.yml
	modified:   engineering_ic_career_matrix.yml
	modified:   event_roadshow.yml
	modified:   event_virtual_devops_workshops.yml
	modified:   events.yml
	modified:   features.yml
	modified:   gdpr_faq.yml
	modified:   heroes.yml
	modified:   heroes_contributions.yml

Related to !53044 (merged) which changes files J-Z.

cc @cwoolley-gitlab because I'm playing in his repo

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • 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.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

For help with failing pipelines reach out in #mr-buddies in Slack