Update changelog for 5.1.0 release.

[mdickinson]

[PR aimed at the release branch; will cherry-pick back into master later]

Changelog entry updates for the 5.1.0 release.

[rahulporuri]

FYI, I had created a milestone to track changes after the 5.0.0 release (but didn't advocate it's use!). I updated the milestone with the PRs mentioned here, after ensuring that they're what are going into the release.

I'll create a similar milestone for after 5.1.0.

[mdickinson]

I'll create a similar milestone for after 5.1.0.

Ah, thank you! Sorry that I missed this.

[mdickinson]

Merged the release branch in to pick up changes in #466 (run CI against non-master branches), will merge when CI signs off.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (release/5.1.x@3af6f5b). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release/5.1.x     #465   +/-   ##
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  Coverage                 ?   65.26%           
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  Files                    ?       44           
  Lines                    ?     7057           
  Branches                 ?     1411           
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  Hits                     ?     4606           
  Misses                   ?     2025           
  Partials                 ?      426

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