REST API: Deprecate active field in POST /runners

What does this MR do and why?

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This MR deprecates the active argument in the POST /api/v4/runners REST API endpoint and adds a paused argument to replace it. The Runner CLI will be adapted accordingly in %15.0 so that we don't break things when a new runner is talking to an older GitLab instance.

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Part of #347211 (closed)