Remove duplicate data view attributes

[lcawl]

Relates to #2778, elastic/kibana#178759

Per https://docs.asciidoctor.org/asciidoc/latest/attributes/names-and-values/ "Although uppercase characters are permitted in an attribute name, the name is converted to lowercase before being stored. For example, URL-REPO and URL-Repo are treated as url-repo when a document is loaded or converted. A best practice is to only use lowercase letters in the name and avoid starting the name with a number". Therefore the "legacy" attributes like Data-Sources are considered to be the same as data-sources and since the former appears later in the attributes file, its value is used in pages such as:

https://www.elastic.co/guide/en/kibana/current/introduction.html

https://www.elastic.co/guide/en/kibana/current/dashboard.html

This PR therefore removes the problematic case-sensitive attribute names.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[joepeeples]

Thanks for opening the PR, @lcawl! I had the same thing on my back burner in #2934 but never fully opened it (got distracted by EAH then 8.13 release).

I added a question about whether we should keep :A-data-source: and :a-data-source: since they're also duplicates, but will give this an approval to speed things along if no further changes are needed.

[joepeeples]

Oh, one other thing: we still have the equivalent case-sensitive variables in the serverless docs at docs.elastic.co.

  "Data-Sources": "Data Views",
  "Data-source": "Data view",
  "data-source": "data view",
  "Data-sources": "Data views",
  "data-sources": "data views",
  "data-source-caps": "Data View",
  "data-sources-caps": "Data Views",
  "data-source-cap": "Data view",
  "data-sources-cap": "Data views",
  "A-data-source": "A data view",
  "a-data-source": "a data view",

They aren't causing trouble like the AsciiDoc attributes are, but we should probably remove them since they're legacy variables that we don't want to encourage anyone to use. Again, I have a draft PR for that, we can clean it up and open it if you want to use that: https://github.com/elastic/docs.elastic.co/pull/205

[joepeeples]

LGTM! I also updated https://github.com/elastic/docs.elastic.co/pull/205 to keep a-data-source in case it's needed on the serverless/Docsmobile side.