fix: add merge_when_pipeline_succeeds webhook field

Giannis Kepasrequested to merge
gitlab-community/gitlab-org/api/client-go:2290-fix-stale-json into main

What does this MR do and why?

Adds merge_when_pipeline_succeeds field to MergeEventObjectAttributes and marks merge_when_build_succeeds for deprecation.

Closes #2290 (closed)

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