ci: Update for Netcode Release Automation

[michalChrobot]

Purpose of this PR

This PR aims to follow up on some issues discovered while using Release Automation for Netcode releases

1. I re-enabled the job trigger that was disabled in ci: Temporarily disabled trigger for release automation #3829
2. I updated pr-description-validation check to not block PRs created by bots (namely prefixed with svc- or unity-renovate
3. I updated regenerate-ci scripts to unify this with N4E approach and made sure that recipes are correctly regenerated for release branch. This is needed because new Editors for validation are fetched dynamically upon recipe regeneration so always worth to double check
4. I decided to modify the action of updating the default branch (Example here) from directly committing changes to creating a PR with them. This helps to address several flaws like:

• We had issues before with branch protection rules that would block the commit from landing due to required checks or reviews
• It will be easier to test this solution or revert it's effects when we don't want to release. Currently we can delete the release branch that was created but to revert the commit we have to push another one. Now if we don't want to proceed with the release OR we want to test the automation by triggering it manually we can just delete the release branch and close the PR to default branch which won't mess around too much

Jira ticket

https://jira.unity3d.com/browse/MTT-12841

Documentation

I will modify Release Automation doc to reflect this change

Testing & QA (How your changes can be verified during release Playtest)

I will run the automation from this branch.

• See created PR and branch in chore: Updated aspects of Netcode package in anticipation of v2.8.1 release #3832
• See green job in https://unity-ci.cds.internal.unity3d.com/job/61350653

Backports

will do