chore: Updated aspects of Netcode package in anticipation of v2.13.1 release

[svc-netcode-sdk]

This PR was created in sync with branching of release/2.13.1. It includes changes that should land on the default Netcode branch (develop-2.0.0) to reflect the new state of the package after the v2.13.1 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

[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@                Coverage Diff                @@
##           develop-2.0.0    #4092      +/-   ##
=================================================
- Coverage          66.44%   66.40%   -0.04%     
=================================================
  Files                163      163              
  Lines              24999    24999              
=================================================
- Hits               16610    16601       -9     
- Misses              8389     8398       +9     

Flag	Coverage Δ
NGOv2_ubuntu_6000.3	66.40% <ø> (-0.04%)	⬇️

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see 3 files with indirect coverage changes

Components	Coverage Δ
com.unity.netcode.gameobjects	66.40% <ø> (-0.04%)	⬇️

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