Upgrade monaco-yaml and re-enable schema validation

What does this MR do and why?

1. Upgrades the monaco-yaml package to 4.0.0
2. Re-enables the yaml schema validation as it seems to be broken for some time

Why to re-enable the schema?

Without it, the users get the error about the schema not being found (when hovering over the first entry in the file)

After the schema re-enabling, things do look like this (when hovering over the first entry in the file):

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #371957 (closed)