chore: Updated aspects of Netcode package in anticipation of v3.1.1 release

[svc-netcode-sdk]

This PR was created in sync with branching of release/3.1.1. It includes changes that should land on the default Netcode branch (develop-3.x.x) to reflect the new state of the package after the v3.1.1 release:

1. Updated CHANGELOG.md by adding new [Unreleased] section template at the top and cleaning the Changelog for the current release.
2. Updated package version in package.json by incrementing the patch version to signify the current state of the package.
3. Updated package version in ValidationExceptions.json to match the new package version.

Please review and merge this PR to keep the default branch up to date with the latest package state after the release. Those changes can land immediately OR after the release was finalized but make sure that the Changelog will be merged correctly as sometimes some discrepancies may be introduced due to new entries being introduced meantime

[u-pr]

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[github_app]
handle_pr_actions = ['opened', 'ready_for_review']  # PR events that auto-run pr_commands
pr_commands       = ["/harness-review"]             # what runs on those events
reviewer_commands = ["/harness-review"]             # what runs when @u-pr is added as reviewer

[u-pr]

💡 Harness Review

The change moves the current release notes under 3.1.1 and advances the package and validation exception versions to 3.1.2. I checked the resulting changelog structure and version references and found no actionable issues.

_{Reviewed commit 8fa16fa}

_{🤖 Helpful? 👍/👎}

[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@              Coverage Diff               @@
##           develop-3.x.x    #4189   +/-   ##
==============================================
  Coverage          76.11%   76.11%           
==============================================
  Files                159      159           
  Lines              27333    27333           
==============================================
  Hits               20805    20805           
  Misses              6528     6528           

Flag	Coverage Δ
NGOv2_project_testproject_ubuntu	75.17% <ø> (ø)

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Components	Coverage Δ
com.unity.netcode.gameobjects	76.11% <ø> (ø)

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