Update and fix external links in Omnibus docs

What does this MR do?

Fixes external links that are broken or redirected, as per the monthly TW chores task.

omnibus/doc/settings/memory_constrained_envs.md
    [ ERROR ] external_links - broken reference to https://www.digitalocean.com/pricing/: link has moved permanently to 'http://www.digitalocean.com/pricing'


omnibus/doc/settings/nginx.md
    [ ERROR ] external_links - broken reference to https://www.digitalocean.com/community/tutorials/how-to-use-apache-http-server-as-reverse-proxy-using-mod_proxy-extension': 404


omnibus/doc/settings/rpi.md
    [ ERROR ] external_links - broken reference to https://www.raspberrypi.org/software/: link has moved permanently to 'https://www.raspberrypi.com/software/'


omnibus/doc/settings/smtp.md
Error: One or more checks failed
    [ ERROR ] external_links - broken reference to https://smtp.com/: link has moved permanently to 'https://www.smtp.com/'
    [ ERROR ] external_links - broken reference to https://www.fastmail.help/hc/en-us/articles/360058753834: 403 
    [ ERROR ] external_links - broken reference to https://wiki.uberspace.de/mail: link has moved permanently to  'https://manual.uberspace.de/'
    [ ERROR ] external_links - broken reference to https://help.sendinblue.com/hc/en-us/articles/209462765-What-is-Sendinblue-SMTP-: 403
    [ ERROR ] external_links - broken reference to https://www.fastmail.help/hc/en-us/articles/360058752854: 403


omnibus/doc/troubleshooting.md
    [ ERROR ] external_links - broken reference to https://packagecloud.io/blog/using-apt-cacher-ng-with-ssl-tls/: link has moved permanently to 'https://blog.packagecloud.io/eng/2015/05/05/using-apt-cacher-ng-with-ssl-tls/'
    [ ERROR ] external_links - broken reference to https://packagecloud.io/blog/apt-hash-sum-mismatch/: link has moved permanently to 'https://blog.packagecloud.io/eng/2016/03/21/apt-hash-sum-mismatch/'


omnibus/doc/update/gitlab_12_changes.md
    [ ERROR ] external_links - broken reference to https://docs.gitlab.com/ee/administration/operations/unicorn.html: link has moved permanently to 'https://docs.gitlab.com/ee/administration/operations/puma.html'


omnibus/doc/update/package_signatures.md
    [ ERROR ] external_links - broken reference to https://turnkeylinux.org: link has moved permanently to 'https://www.turnkeylinux.org/'

Related issues

technical-writing#607 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.