[changelog-generator] Group unknown entries by author

[alloy]

When changelog entries cannot be attributed to the correct category, they are put in an “unknown” section. Grouping these by author would make it easier to ask point of the authors to the entries we need addressed.

[alloy]

After discussing more with @TheSavior, the assumption is that this need will largely go away in v0.63.0, so we’ve agreed to do the sorting as a one off in the simplest way possible for just the v0.62.0 release.

[alloy]

#166 (comment)