Add condition for 401 on invalid key and changelog on API Guide

[jp-tosca]

What this PR does / why we need it:

• Add a section to the guide titled "Api Changelog" and add the mentioned change in Bug or feature? - Invalid api token results in access denied to public objects #10070 as a 6.0.0 change.
• Add a condition to AccessIT to check for a 401 when providing a null key.

Which issue(s) this PR closes:

• Closes: Bug or feature? - Invalid api token results in access denied to public objects #10070
• Closes: section in API Guide for changes in behavior and backward incompatibilities  #10060

Special notes for your reviewer:
n/a
Suggestions on how to test this:
n/a
Does this PR introduce a user interface change? If mockups are available, please link/include them here:
n/a
Is there a release notes update needed for this change?:
Probably we could add that now we have a changelog for the API in the general notes.

Additional documentation:

https://dataverse-guide--10127.org.readthedocs.build/en/10127/api/changelog.html

[coveralls]

coverage: 19.996%. remained the same
when pulling 70edaa7 on jp-tosca:10070-api-doc-changelog
into 8d8f35f on IQSS:develop.

[pdurbin]

I left some feedback.

Can we have this PR also close the following issue?

• section in API Guide for changes in behavior and backward incompatibilities  #10060

[pdurbin]

A tiny comment on style/formatting.

[pdurbin]

I added more detail as a suggestion.

[pdurbin]

In 640f69e I added a release note snippet about the new API changelog.

[pdurbin]

Tests are passing: https://jenkins.dataverse.org/job/IQSS-Dataverse-Develop-PR/job/PR-10127/4/testReport/

@landreev and @scolapasta are good with the changes as well. Merging.

[pdurbin]

Looks good.