REST API: deprecate old pull mirror configuration process

Vasilii Iakliushinrequested to merge
494294_deprecate_old_pull_mirror_configuration into master

What does this MR do and why?

Contributes to #494294 (closed)

!167901 (merged) introduced a dedicated API to configure pull mirrors. It's a recommended way to update them.

The previous configuration via Projects API is discouraged and deprecated.

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Edited by Vasilii Iakliushin

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