Document prometheus metric deprecations

[wozniakjan]

Original work in #1258, reasons for duplicate PR kedacore/keda#5687 (comment)

Updates the docs with the changes in kedacore/keda#5174.

I merely cherry-picked the documentation changes and applied review suggestions

• moved the docs to target 2.14
• fixed minor typos
• also kept original text next to the deprecated metrics right after the deprecation notice and recommendation to use new metric

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)

closes: #1258

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[wozniakjan]

thanks @tomkerkhove, I applied your suggestions, let me please know if there is anything else.