Update custom dashboard docs

[angorayc]

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.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @angorayc? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.