Backport of "Stop logging GraphQL type conflict deprecation warnings"

What does this MR do and why?

Backport of !250883 (merged) to 19.3.

GraphQL-Ruby logs a three-line deprecation warning every time a query selects one response key with two different types. 26 of GitLab's own frontend queries do this, so instance logs fill up with warnings that make real incidents harder to spot. One customer reported 102 entries in 15 minutes.

The fix opts the schema into the legacy behaviour and counts the conflicts instead. Queries keep running exactly as before, and the per-request warning is replaced by the gitlab_graphql_return_type_conflicts_total counter so the remaining offenders stay visible.

Requested by @dnsmichi in #586994 (comment 3776011436), on behalf of several self-managed customers reporting log noise.

The cherry-pick applied cleanly, so this is identical to the original commit.

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.
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• 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.