Add change log entries for PublishedMetadata change

[dkliban]

re: #5304
https://pulp.plan.io/issues/5304

[codecov]

Codecov Report

Merging #133 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #133   +/-   ##
=======================================
  Coverage   42.56%   42.56%           
=======================================
  Files          22       22           
  Lines         740      740           
=======================================
  Hits          315      315           
  Misses        425      425

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[bmbouter]

There is one line like this in the docs that needs an update: PublishedMetadata is a repository metadata which is published, located in /var/lib/pulp/published and belongs to a certain Publication.

I saw it from this query: https://docs.pulpproject.org/en/pulpcore-plugin/nightly/search.html?q=PublishedMetadata&check_keywords=yes&area=default

[bmbouter]

Also this example uses the default constructor: metadata = PublishedMetadata(...) https://docs.pulpproject.org/en/pulpcore-plugin/nightly/api-reference/models.html?highlight=publishedmetadata

[dkliban]

I pushed the changes you suggested. One of them had to be made in the pulpcore PR.

[bmbouter]

This looks good; thank you for doing the additional docs.