describe-changes@v1: with concatenate mode: append

✨ PR Description

Purpose: Update gitstream-core version to 2.1.207 and improve GitHub release workflow with conditional changelog handling.
Main changes:

• Updated default gitstream-core version from 2.1.133 to 2.1.207 in workflow configuration
• Added conditional logic to use custom changelog when available or generate release notes automatically
• Fixed PR title formatting with proper escaping of backticks for markdown rendering

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