Update notification-integrations.mdx

[dkoyano]

The Slack Integration install instructions lead the customer to believe they should contact NR support for assistance with approval of the application before they can individually install it. They should contact their internal Slack admin.

Please follow conventional commit standards
in your commit messages and pull request title.

Give us some context

• What problems does this PR solve?

The documentation leads the customer to believe they should contact NR support with their internal Slack permissions and provides instructions on how to create a case for this. This should be removed as support cannot help with this.

• Add any context that will help us review your changes such as testing notes,
  links to related docs, screenshots, etc.
• If your issue relates to an existing GitHub issue, please link to it.

[github-actions]

Hi @dkoyano 👋

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[adutta-newrelic]

netlify build fork

[svc-docs-eng-opensource-bot]

✅ Your PR has been mirrored to our repository as PR #21850.
Commit: 091c9da3abd775fe5dff0cef4430401506fc93d9 (091c9da)
Our workflows will run in the mirrored PR linked above.
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