Streamline Integrations page titles

What does this MR do and why?

To pare down page titles under https://docs.gitlab.com/ee/integration/, this MR:

• Deletes integration or service from the titles.
• Refers to each integration by its name at https://gitlab.com/gitlab-org/gitlab/-/settings/integrations whenever possible.

I'll open an MR in the gitlab-docs repo to update the navigation entries for the affected pages when this MR is merged.

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.