[8.x] [Data Views] Backport Upgrade Assistant changes to warn about scripted field creation removal in 9.0

[davismcphee]

Summary

This PR backports the Upgrade Assistant changes from #202250 to warn about the upcoming scripted field creation removal in 9.0. It also includes some of the documentation changes and cleanup in the Data Views Management UI.

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines

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