[8.17] Update custom dashboard docs (backport #6470)

[mergify]

Context: https://discuss.elastic.co/t/unable-to-add-tag-security-solution-in-elastic-security-serverless/373206

The default "Security Solution" tag is now a managed tag, so users won't be able to assign it to a dashboard.
Suggesting to remove the tips in the document to avoid confusion.

---

This is an automatic backport of pull request #6470 done by [Mergify](https://mergify.com).

[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.}