AI-Aided: Add data retention support for preserving pinned posts

[cwarnermm]

Add documentation for the new data retention feature that optionally preserves pinned posts, available from Mattermost server v10.10 onward via mattermost/mattermost#31165

Changes

• Add Preserve pinned posts configuration setting in compliance settings
• Update data retention policy documentation to reflect pinned post preservation option
• Document config.json setting and environment variable

Closes #8070

Generated with Claude Code

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 082ca2b

[cwarnermm]

@claude - Concisely explain why this PR contains changes to other parts of the repository beyond what was asked.

[claude]

Claude finished @cwarnermm's task —— View job

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Analysis of PR Changes

Investigating why this PR contains changes beyond the data retention feature scope.

Todo List:

• Read data retention related files to understand core changes
• Read mobile app changelog/release files to understand those changes
• Read subscription and deployment files to understand those changes
• Categorize changes and identify what's unrelated to the original task
• Provide concise explanation

Analysis Complete

The PR contains changes to 6 files, but only 2 are directly related to the data retention feature for preserving pinned posts:

Related to Issue #8070 (Data Retention):

• source/comply/data-retention-policy.rst - Added documentation for the preserve pinned posts feature
• source/configure/compliance-configuration-settings.rst - Added configuration setting documentation

Unrelated changes that were included inadvertently:

• source/about/mattermost-mobile-releases.md - Mobile app release table updates
• source/about/mobile-app-changelog.md - Mobile app v2.29.1 changelog entries
• source/about/subscription.rst - Subscription documentation updates
• source/deploy/server/deploy-linux.rst - Linux deployment documentation changes

Explanation

This PR contains changes to other parts of the repository because the branch was created from a point that included recent commits for mobile releases (#8060), subscription updates (#8069), and Linux deployment reorganization (#8025). When the data retention feature work was added on top of these commits, all the changes became part of this PR.

The branch should have been created from a clean main/master branch state containing only the data retention changes, or the unrelated commits should have been excluded from the PR scope.

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