docs: Remove the instruction for Ruby version for Ruby 1.9 era

What does this MR do?

Removes the instruction for Ruby version for Ruby 1.9 era from the docs. That should be removed after %11.7 (2019-01-22).

Debian

Debian 7 (wheezy) (which reached EOL by 2018-05) included ruby1.8 package according to https://wiki.debian.org/Ruby, and Debian 8 (jessie), which was also deprecated from the supported OS of GitLab, dropped it.

Ubuntu

Ubuntu 14.04 did not include ruby1.8 package as it had ruby1.9.1 by default, while Ubuntu 12.04 (which reached EOL by 2017-04) did not include ruby1.8 package as it had ruby1.9.1 by default.

Source

This instruction was introduced in d58aca06 on 2013-05-24. https://github.com/gitlabhq/gitlabhq/commit/d58aca0693d28e17633fb0d2825f1cddfd4f0748

Related issues

n/a

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Request a review from the designated Technical Writer.
• @tnir hopes this in time for %13.12, but if it will not be, backport this to 13-12-stable-ee.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Ensure docs metadata are present and up-to-date.
  • Ensure Technical Writing and documentation are added.
  • Add the corresponding docs:: scoped label.
  • If working on UI text, add the corresponding UI Text scoped label.
  • Add twdoing when starting work on the MR.
  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

For more information about labels, see Technical Writing workflows - Labels.

For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.