Fix minimum token permissions for workflows that perform git push

[Copilot]

This PR adds the required contents: write permission to workflow jobs that perform git push operations, ensuring they have sufficient permissions to push changes to the repository.

Changes Made

Added contents: write permissions to 8 workflow jobs across 6 workflow files:

prepare-release-branch.yml:

• create-pull-request-against-release-branch job
• create-pull-request-against-main job

package-prepare-release.yml:

• create-pull-request-against-release-branch job
• create-pull-request-against-main job

package-prepare-patch-release.yml:

• prepare-patch-release job

backport.yml:

• backport job

package-release.yml:

• release job

prepare-patch-release.yml:

• prepare-patch-release job

All permissions include the required trailing comment # required for pushing changes as specified in the issue requirements.

Validation

• ✅ All workflows with git push operations now have contents: write permissions
• ✅ YAML syntax validated for all modified files
• ✅ Permissions added only to jobs that actually perform git push operations
• ✅ No unnecessary permissions added to other jobs

Fixes #30.

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