Update Slack notifications integration UI text

What does this MR do?

Resolves #300622 (closed)

Updates the UI Text for the Slack integration. Only a few tweaks were needed as the majority of the fields were updated in !58570 (merged) (shared between all chat notification integrations). I only took screenshots of the top of the integration, as everything else is unchanged (shared with the Mattermost integration, so fixed in that MR. MR)

Screenshots (strongly suggested)

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