ci: Adjust changelog format

[EttyKitty]

Summary by cubic

Removed extra newline after the changelog in the Release template and renamed 'Full Changelog' to 'Full Diff' for cleaner release notes.

^{Written for commit 1c02e0e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

_{Re-trigger cubic}