[8.15] Fix screenshots for Nginx integration, diagnostics, & failed upgrades docs (backport #1246)

[mergify]

Fixes outdated screenshots for:

• the two Nginx ingest tutorials.
• the diagnostics and failed upgrade docs

Closes: #1104
Closes: #1105

---

This is an automatic backport of pull request #1246 done by Mergify.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}