[19.4] Backport of 'Update oj to 3.17.8 to fix native memory-safety bugs'

What does this MR do and why?

Backport of !268704 (merged) to the 19.4-stable-ee branch.

Updates oj from 3.17.6 to 3.17.8. oj 3.17.8 fixes roughly 20 native memory-safety bugs, including several segmentation faults, use-after-frees, and memory corruption issues. See the oj changelog (3.17.8).

Motivation

GitLab.com incident INC-15953 saw ~427 Ruby worker segmentation faults on a single webservice pod, generating ~16,900 HTTP 502s. All crashes were on the notes/discussions request path, which serializes a large amount of JSON through oj. This bump is the leading mitigation for that incident.

Cherry-pick

The default-branch merge commit could not be cherry-picked cleanly because this branch ships oj 3.17.6, not the 3.17.7 the MR bumped from. The change was reapplied by hand: it is the same mechanical Gemfile.lock / Gemfile.next.lock edit (version bump, checksum update, and removal of the oj-only ostruct dependency) that landed on master.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.