[8.14] [Request][ESS] Update ES|QL rule type metadata operator syntax (backport #5167)

[mergify]

Contributes to #5150

Preview:

• Create a detection rule| Create an ES|QL rule - Updated the METADATA syntax that was used in examples throughout this section.
• Create rule API - Two ES|QL rule examples that included the METADATA field updated (thanks, @vitaliidm!)

Serverless docs updated in https://github.com/elastic/staging-serverless-security-docs/pull/340

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This is an automatic backport of pull request #5167 done by Mergify.

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