Add missing deprecation notices
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This merge request updates the GitLab provider documentation and code to mark several project configuration options as deprecated. The changes add clear deprecation warnings to fields like issues_enabled, merge_requests_enabled, wiki_enabled, snippets_enabled, and others, indicating they will be removed in version 19.0. For each deprecated field, the documentation now specifies which newer field should be used instead (for example, use issues_access_level instead of issues_enabled). The changes also standardize the deprecation messages to be more consistent and informative, helping users understand what alternatives to use and when the deprecated features will be removed. This is part of modernizing the GitLab API integration by moving from simple boolean enable/disable options to more granular access level controls.