[PPV2] Deprecating the package as Birp is being deprecated.

[alex-vazquez-unity3d]

Please read the Contributing guide before making a PR.

• Read the Graphics repository & Yamato FAQ.

Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the
  PR, the label changes to backported-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or
  change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you
  do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not
  sure which person to add, see the Docs team contacts contacts
  sheet.
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests
  don't cover, please add a graphic test.

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Purpose of this PR

Officially deprecate Post Processing Stack v2 (com.unity.postprocessing) package.

This package is no longer actively developed as Unity's rendering pipelines (URP/HDRP) now have integrated
Volume-based post-processing systems. This PR adds comprehensive deprecation notices across all user touchpoints:

1. Package Manager UI - Displays "Deprecated" tag and warning message
2. Startup Dialog - Notifies users once per session with migration guidance
3. Compiler Warnings - [Obsolete] attributes on core APIs (PostProcessVolume, PostProcessLayer,
   PostProcessProfile)
4. Documentation - Enhanced deprecation banners with migration paths

Key technical changes:

• Added deprecated field to package.json with migration guidance
• Suppressed CS0618 warnings in package-internal code to prevent false positives
• Added startup dialog with "Don't show again" option using Unity's built-in opt-out system

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Testing status

Manual Testing:

• ✅ Verified deprecation dialog appears once per Unity session
• ✅ Confirmed "Don't ask again on this computer" checkbox permanently dismisses dialog
• ✅ Tested compiler warnings appear when using PostProcessVolume, PostProcessLayer, PostProcessProfile
• ✅ Verified package internal code compiles without CS0618 warnings
• ✅ Confirmed documentation displays deprecation banner

Modified Modules:

• com.unity.render-pipelines.universal/Editor/Tools/Converters/PPv2/PPv2Converter.cs - Suppressed CS0618 for PPv2
  converter tool

No graphic tests needed - This is a deprecation notice feature, not a rendering feature change.

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Comments to reviewers

For Graphics Docs team: Please review the documentation changes:

• com.unity.postprocessing/Documentation~/snippets/birp-deprecation-message.md - Enhanced deprecation banner
• com.unity.postprocessing/CHANGELOG.md - Added [Unreleased] section (version number TBD by release management)

Version strategy:

• Version number left as 3.5.4 in package.json for release management to decide
• CHANGELOG uses [Unreleased] section
• Suggest bumping to major version (e.g., 6.x.x) to clearly signal deprecation, but defer to release management

[theo-at-unity]

LGTM but I will let @BenGraterUnity chime in about the questions you had on Slack.