Undocumented changes on user assignable searches API endpoints

Issue Summary

In Jira 8.20.7, we introduced a change on these REST API endpoints due to this bug report:

user/assignable/search
user/assignable/multiProjectSearch
user/viewissue/search
user/mention/search

Where these endpoints don't support "startAt" parameter anymore,
and in these endpoints, the "maxResults" parameter cannot exceed 100.

This is breaking customers' scripts and integrations.

Steps to Reproduce

Upgrade to Jira 8.20.7

Expected Results

To communicate this change on our release notes:

• Issues resolved in 8.20.7

Actual Results

Customers are upgrading without knowing of this change and having to fix or modify their scripts.

We should at least add this change to our release notes Issues resolved in 8.20.7

Workaround

Modify scripts to not use the "StartAt" parameter