Fix block scalar newline interpretation in release workflow

[adamtheturtle]

The previous fix used a YAML block scalar (|-) which doesn't interpret \n as newlines, causing literal \n to appear in CHANGELOG.rst. This fix uses a double-quoted string on a single line which correctly interprets escape sequences.

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Note

Ensures correct newline handling when inserting the release header into the changelog.

• In .github/workflows/release.yml, the "Update changelog" step now uses a double-quoted replace string so \n escapes are interpreted as newlines when updating CHANGELOG.rst.

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